Politicians, senior police officers and members of the public have gathered in Trafalgar Square for a candlelit vigil for Wednesday's attack.
See our slide of pictures below.
This concludes the live blog of the aftermath of Wednesday's attack in Westminster. For the latest developments, check the IBTimes UK homepage.
The Met Police has revealed the latest on the investigation into Khalid Masood's attack.
It said detectives are continuing to search a number of addresses linked to the investigation:
- One in Carmarthenshire;
- Three in Birmingham;
- One in east London.
Searches at addresses in Brighton and south east London have concluded, detectives added.
Here is the latest on arrests made last night:
- A woman, 39, was arrested at an address in east London on suspicion of preparation of terrorist acts.
- A woman, 21, and a man, 23, were arrested at an address in Birmingham on suspicion of preparation of terrorist acts.
- A woman, 26, and three men aged 28, 27 and 26 were arrested at a separate address in Birmingham on suspicion of preparation of terrorist acts.
A man, 58, was also arrested on this morning at a separate address in Birmingham on suspicion of preparation of terrorist acts.
A Met Police Spokesman said: "The investigation by the Met's Counter Terrorism Command into the terrorist attack on Wednesday continues to develop at a fast pace and involves hundreds of detectives."
The police watchdog has opened an investigation into the circumstances of the fatal shooting of attacker Khalid Masood, as is standard procedure in incidents of this kind.
The Independent Police Complaints Commission (IPCC) stressed no police officers were under investigation.
IPCC deputy chair Sarah Green said: "The terrorist attack in Westminster on Wednesday was horrific and our thoughts are with all of those affected by it.
"The IPCC is responsible for investigations into fatal police shootings. Accordingly, after being notified by the Metropolitan Police Service of the fatal shooting of a man outside the Houses of Parliament, we have opened an independent investigation.
"It's important to stress that we are currently investigating the circumstances of the incident and no police officers are under investigation.
"The IPCC and the Metropolitan Police Service are following agreed working arrangements related to terrorist incidents, which are working well. I recognise that this is a very difficult time for the police service, following the tragic loss of PC Keith Palmer and the injuries to other officers. We are giving this investigation the highest priority and we will conclude it as soon as possible."
Berlin's iconic Brandenburg Gate has been illuminated with the colours of the British flag in solidarity with the victims of the Wednesday's attack.
Angela Merkel, the German chancellor, had expressed her shock at the attack.
She said "we stand firmly and resolutely by Britain's side when it comes to fighting any kind of terrorism".
People have started to gather for this evening's candlelit vigil in Trafalgar Square, led by Mayor of London Sadiq Khan.
Some members of the public have been drawing messages of solidarity on the pavement with chalk.
The vigil starts at 6pm and is expected to last one hour. You can read more here.
King's College hospital, one of the major trauma centres treating some of the injured victims from Wednesday's attack, has given an update on their condition.
Two patients have been discharged, four are stable and two are in a critical condition.
Home Secretary Amber Rudd has told the BBC it would be "absolutely the wrong judgement" to see Wednesday's attack as intelligence failure.
She also warned against a "knee-jerk" reaction, furthering speculation the government does not consider it necessary to bring forward a policy change or introduce fresh anti-terror legislation.
Far-right political party Britain First has been condemned by anti-extremism campaigners for planning a march outside Parliament next Saturday (1 April) in the wake of Wednesday's attack, IBTimes UK reports.
It comes amid concerns from police over the activities of right-wing extremists in the wake of the attack.
Family tribute to Keith Palmer
The family of PC Keith Palmer have just paid tribute to the 48-year-old following his death, describing him as a "wonderful dad and husband".
The family added:
"Keith will be remembered as a wonderful dad and husband. A loving son, brother and uncle. A long-time supporter of Charlton FC.
"Dedicated to his job and proud to be a police officer, brave and courageous.
"A friend to everyone who knew him. He will be deeply missed. We love him so much.
"His friends and family are shocked and devastated by his loss and ask that they are left to grieve alone in peace."
Met Police have confirmed that as a mark of respect, Palmer's shoulder number - 4157U - will be retired and not reissued to any other officer.
Palmer joined the force in 2001 and was nominated in 2015 for best thief taker in the Commissioner's Excellence Awards, having made more than 150 arrests in 12 months.
PC James Aitkenhead, who worked alongside Keith in the TSG, said:
"Keith was a genuinely nice person; nobody had a bad word to say about him. When I heard what had happened I knew it would be him because that's just the sort of guy he was, to step straight in when others might step back.
"He had a great work ethic, he worked on our warrants' car for years, getting up at 4am to serve warrants and arresting wanted offenders. He was always so positive, always staying late after everyone else and getting in early.
"In his personal life he was a massive Charlton Athletic fan and had a season ticket. We will miss him so much."
Key points to take from what police have said is that the suspect does have previous convictions for crimes such as GBH and possession of a knife, but has never been convicted of a terrorist offences.
Police also say he was not under any investigation and there was no intelligence to suggest he might commit a terrorist attack.
Sky News are reporting that Masood was a married father-of-three, worked as an English teacher and was in to body building.
Full story here.
Terror suspect named
Police have named the suspect in yesterday's attack as 52-year-old Khalid Masood.
A statement added:
The man police believe to be responsible for the terrorist attack in Westminster yesterday, Wednesday, 22 March, has now been formally identified as Khalid Masood.
Masood, aged 52 (25.12.1964), was born in Kent and detectives believe he was most recently living in the West Midlands. Masood was also known by a number of aliases.
Masood was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack.
However, he was known to police and has a range of previous convictions for assaults, including GBH, possession of offensive weapons and public order offences.
His first conviction was in November 1983 for criminal damage and his last conviction was in December 2003 for possession of a knife.
He has not been convicted for any terrorism offences.
Anyone with any information about Masood can call the Anti-Terrorist Hotline 0800 789 321.
The JustGiving fundraiser for Keith Palmer, the on-duty officer killed in yesterday's attack, has just hit £100,000.
You can donate here.
Exactly 24 hours after the attack first began, London appears to be operating at almost full capacity.
Transport for London (TfL) have confirmed that well as Westminster Bridge being re-opened to the public, the nearby Tube station has also returned to a full service.
Here we have some pics showing police patrolling the capital the day after the attack in which on-duty officer Keith Palmer was killed.
Fox News have claimed that a man from Utah is among the dead in the Westminster terror attack London.
Kurt Cochran and his wife Melissa, who sustained serious injuries, were reportedly caught up in the attack.
He is now the third victim to be formally identified.
Full story can be read here.
The junction between Old Street, Shoreditch High Street and Hackney Road was been cordoned off by the police while officers dealt with an "an unattended item". It later tuned out to be a false alarm.
A Met Police spokesperson said: "Police were called at 13:24pm on Thursday, 23 March, to a suspicious object in Curtain Road near to Old Street and Kingsland Road.
"Officers attended the location and cordons were put in place. The incident was stood down shorly before 2pm."
Police have confirmed that Westminster Bridge has re-opened less than 24 hours after Wednesday's attack.
At least two people were killed after dozens more injured after a vehicle drove into pedestrians before crashing into gates at the Houses of Parliament.
A woman who attracted media attention following the attack after being pulled from River Thames after falling from Westminster Bridge has been named.
Romanian national Andreea Cristea survived but remains seriously injured after receiving medical treatment on the side river in the wake of the attack.
It is not known if she was thrown from the bridge after being struck by the vehicle or if she jumped to get out of the way.
Some have noticed the wording of the statement could mean the group did not plan the attack themselves, more the attacker was responding to their ideology.
The Jihadist terror group added:
The perpetrator of the attacks yesterday in front of the British parliament in London is an Islamic State soldier and he carried out the operation in response to calls to target citizens of the coalition.
The full story on the latest claims from Isis can be read here.
Isis have claimed responsibility for the attack in which four people died, describing the terrorist as a "soldier of Islamic State" according to the Amaq news agency.
Theresa May previously confirmed in the House of Commons police are working under the assumption the attacker was "inspired by Islamist ideology".
No other details have been released about the assailant who was shot dead outside parliament expect he is British-born and previously investigated by MI5 for violent extremism but considered "peripheral" and "not part of the intelligence picture".
Our reporter Ian Silvera is currently in London getting reaction from the public on yesterday's attack.
Charlton football club have paid tribute to PC Keith Palmer, a "loyal Charlton supporter and season-ticket holder" who was killed on-duty in yesterday's attack.
Charlton have also released an imaged of a scarf on the seat in which the officer "occupied for many years" as part of the tribute.
In a statement, the club added:
"It is with great sadness that the club has learned of the tragic news of the death of PC Keith Palmer, a loyal Charlton supporter and season-ticket holder, who lost his life as a result of the terror incident in Westminster on Wednesday afternoon.
"Keith was a familiar face at The Valley to many supporters and sat in his same East Stand seat for many years.
"As an immediate tribute, a red and white scarf has been placed on his seat which will remain until the next home game on Tuesday, April 4th, while the club will discuss ways in which it can commemorate his life at the game itself.
"Keith was a true hero who will be greatly missed by all the Charlton family and everyone at the club would like to offer their sincere condolences to his family and friends at this extremely difficult time.
"The club also wish to extend condolences to the families of all those victims who were involved in yesterday's incident."
Tributes have also continued to pour in from across the world. In India, world famous sand artist Sudarshan Pattnaik has created this sculpture condemning the attack on a beach Puri Beach in the state of Odisha.
The Queen has said her "thoughts, prayers and deepest sympathies" with all those affected by yesterday's attack.
In a message to acting Met Police Commissioner Craig Mackey, she added:
Following yesterday's shocking events in Westminster, Prince Phillip and I are sorry that we will not be able to open the new Scotland Yard building as planned today, for very understandable reasons. I look forward to visiting at a later date.
My thoughts, prayers and deepest sympathies are with those who have been affected by yesterday's awful events.
I know I speak for everyone in expressing my enduring thanks and admiration for the members of the Metropolitan police service and all who work so selflessly to help and protect others.