Hijacked United Airlines Flight 175
Hijacked United Airlines Flight 175 (L) flies toward the World Trade Center twin towers

On a day that should have been remembered for the memory of the 9/11 terrorist attacks on New York and Washington ten years ago, two extremist groups stole the headlines in London after a battle on the streets of London broke out during the 9/11 memorial.

Dozens of people were arrested as members of the English Defence League and Muslims Against Crusades clashed during the memorial service that was attended by Prince Charles, the Duchess of Cornwall as well as Prime Minster David Cameron, Deputy Prime Minister and leader of the Liberal Democrats Nick Clegg and leader of the opposition Ed Miliband. The Mayor of London Boris Johnson also attended, and abuse was hurled at him by both groups as he entered.

Supporters of both the English Defence and Muslims Against Crusades clashed outside the U.S. Embassy in London only metres away from where the 67 British victims of the 9/11 terrorist attacks were mourned by relatives.

The Metro reported Monday that one of the Grosvenor Square memorial service attendees, who did not want to be named, said the protesters should have been stopped from standing just across the road from the embassy and using a loud megaphone.

"They shouldn't be allowed to do it. It's very disrespectful. It's too loud," the man, whose cousin died in the terror attacks, said.

"They can say what they want but not with the loudspeaker," he added.

Four people, two from each side were arrested on Grosvenor Square, Scotland Yard confirmed before the Metropolitan police detained several more Islamists and around 20 EDL supporters the Metro has reported.

The Evening Standard reported Monday that two men were seriously injured when they were stabbed in the clashes. The newspaper reports that the two men were on opposition sides during the clashes after a street battle was carried out outside a pub on Edgware Road.

"There was a group of EDL guys drinking outside the pub when a large group of Muslims walked past. The EDL started hurling racist abuse at the Muslims, who took offence. "They got angry and were yelling back," an eyewitness to the battle, who did not wish to be named by the Evening Standard, said

"Then the EDL guys picked up pub furniture and started throwing glasses and chairs and tables at the group in the street. It was chaos. At first it was just 10 EDL guys but that number quickly swelled to about 50 people," he added.

Despite the violence outside, the memorial service was a moving day for the relatives of the 67 British victims of 9/11.

The Daily Mirror reported Monday that Science journalist Tom, 38, from Islington, North London, whose sister died in the attacks, was not going to allow the protestors to ruin the day.

"I mourn my sister's loss every day but the run-up to this anniversary has brought so many images and reminders of just how horrific and prolonged my sister's death was," he said.

"That's something you don't come to terms with. Because we never had my sister's remains to bury, we don't know exactly what happened to her. But we do know, tragically, that she was in the worst place you could be, trapped from any possible hope of escape," he added.

Prince Charles gave a heartfelt speech in which he referred to his own devastation and struggle to find forgiveness after his Uncle Lord Louis Mountbatten was killed in an IRA terror attack in 1979.

"None of us will ever forget where we were or what we were doing when, on that otherwise ordinary day and out of a clear blue sky came so much premeditated death and destruction on a scale that shook the whole world," he said in a moving speech.

Mourners were determined to honour those who were killed in the terrorist atrocities and to honour the lives their relatives led.