Hundreds of bikers brought three major UK cities to a standstill during a chaotic "Halloween ride out" which was condemned by police as "loutish, criminal behaviour".
The events, orchestrated on social media, saw riders perform dangerous stunts on busy streets, speed along pavements, attack motorists with fireworks and smash up cars.
Officers, who made only a handful of arrests, say they are now hunting for those involved, vowing to bring them "to book".
In London, a large group of people rode en masse on motorcycles and quad bikes from the south-east suburb of Blackheath through Oxford Street in the city centre on Sunday night (30 October).
Witnesses said pedestrians were forced to dive for cover as bikers mounted pavements and threw firecrackers at police and people's windows.
The Met Police was forced to deploy stingers to stop the bikers, bursting the tyres of 17 motorcycles and three quad bikes. Officers later arrested seven men and two boys for offences including dangerous driving, failing to stop and theft.
The following night saw traffic brought to a standstill in Birmingham and Solihull as another gang of about 200 motorcyclists – many wearing masks – took part in a similar "take over" event.
Riders sped through the city's streets doing wheelies in front of oncoming traffic, jumping red lights and weaving between motorists.
At one point about 100 bikes surrounded a police van in Chelmsley Wood, on the outskirts of the city, where they reportedly kicked at its wing mirrors and tried to smash the vehicle's windows. They sped off as police back-up arrived.
The night also saw one motorcyclist – believed to be riding without lights or registration plates – suffer life-threatening chest injuries after being involved in a collision with a car in Solihull.
West Midlands Police, which branded the ride out as "outrageous and totally unacceptable", were again forced to use stingers to stop the bikers, with 10 bikes eventually seized. Five men and two male youths were arrested on suspicion of dangerous driving, causing a public nuisance and motoring offences.
And the same night saw about 50 off-road motorcycles and quad bikes cause havoc on the streets of Leeds in another ride out event organised on social media. Witnesses posting videos of the disturbance on Twitter described the scenes as "utter chaos" and likened them to the film Mad Max.
Riders were again condemned by police for performing "dangerous" stunts, with footage posted online showing pedestrians sheltering as motorcyclists sped down a pedestrianised area. Kirkstall Road, a major route through the city, was closed for around 90 minutes as a result, with a police helicopter also called.
West Yorkshire Police said the group had dispersed by about 10pm, adding that it had arrested a 26-year-old man.
"Real bikers" took to social media to distance themselves from the incidents. Martin Swingewood said: "I'm a biker and these are just idiots racing around trying to be big and making themselves look like the idiots they are. As for 20-year-olds riding any bike they want, well they can't as the law requires a series of tests to be taken and at 20 you're restricted in power and the size of the bike.
"These aren't bikers – they are pests and idiots."
Peter Byrne added: "These are not true bikers. They are a disgrace endangering the lives of the general public, as well as themselves, riding recklessly on illegal bikes."
Superintendent Ade Adelekan, of the Met Police, said of the disturbance in London: "The recklessness of the riders involved endangered not only themselves, but other road users and pedestrians. This is totally unacceptable behaviour that tied up significant resources that could have been better employed in emergency situations elsewhere."
Chief Superintendent Paul Money, of West Yorkshire Police, said the disturbance in Leeds was "unacceptable".
He said: "It is no secret that we are generally experiencing significant levels of demand on our limited resources and with Halloween last night it was one of our busiest nights for calls for service. What we did not need was the mindless actions of a few creating additional unnecessary demand on those limited resources and potentially causing delays to members of the public in need of a police response."
And West Midlands Police Chief Inspector Jack Hadley described the bike gathering – known as a Halloween Ride Out – as "dangerous and criminal". He said: "This wasn't a rally of bike enthusiasts – this was loutish, criminal behaviour. Bikers were riding without lights, at speed, dangerously, pulling wheelies in the street, mounting pavements and damaging vehicles.
"We had one call from fire service colleagues reporting a group of bikers stopping a fire truck from responding to an emergency call. We will be doing everything we can and using all available criminal and anti-social behaviour legislation to bring those involved to book."