Riots broke out at the scene of a crash in Cardiff as cars were set alight and fireworks were thrown at police.
The “serious” traffic collision is understood to have left two teenagers dead, with police facing “large-scale disorder” following the crash.
Police and Crime Commissioner in South Wales, Alun Michael, said a fatal road accident appeared to have “sparked” the rioting.
He told BBC Radio 4’s Today programme: “My understanding is that there was a road traffic accident involving two teenagers on an off-road bike or scooter and sadly, they died. That’s an incident obviously that is being investigated in its own right.
“And that appears to have sparked, for reasons that aren’t clear, the disorder… in which something like a dozen officers were injured.
“Fortunately, none of them life-threatening, and the connection between the two is far from clear.
“So obviously there’s going to be investigations going on this morning to try and establish what happened.
“It would appear that there were rumours, and those rumours became rife, of a police chase, which wasn’t the case and I think it illustrates the speed with which rumours can run around with the activity that goes on social media nowadays, and that events can get out of hand.”
Officers were called to the scene of the crash in Ely just after 6pm on Monday, with rioters still on the streets nine hours later.
Pic: PA
At least two cars were set on fire and rioters threw missiles, including fireworks, at a line of police officers with riot shields blocking one end of the street.
A fire was burning and a helicopter was hovering overhead.
A member of the public was also attacked due to rioters thinking they were an undercover officer, police said.
John Urquhart, general secretary of the UK Harmony Party, who lives in the area, claimed a lack of communication from police to the community may have led to the escalation.
“They showed nothing but disdain for the community and acted like we didn’t deserve to know what happened on our own doorstep,” he said.
Police in riot gear at the scene of the disorder
Jane Palmer, owner of a burnt out Ford Focus on Highmead Road, said she and her family had watched from their window as rioters set fire to her car.
Ms Palmer said: “I’m disabled, so now I’m trapped without my car.
“Why are they doing this? It’s just silly now.”
The family had tried to stop the fire using water from their garden hose.
At 8.21pm, South Wales Police said it had a “large number of officers working to manage the collision, but also to de-escalate ongoing disorder at the scene”.
“We urge anyone involved to leave the scene immediately and ask local residents to stay away while the matter is brought to a safe conclusion,” they added.
At 1.10am, they said a “large police presence” remained in the area, confirming that vehicles had been set on fire and people had been arrested.
One resident of Highmead Road, who did not want to be named, said: “Enough is enough now. This has gone beyond.
“When people are making fun of what’s happening on social media it’s crossed a line.
“This is our neighbourhood and those doing this are just kids. It needs to stop.”
A man called Connor, who also lives in Highmead Road, said: “It has been a mad night.
“This is not the norm around here but people are pretty anti-police and that’s reflected on both sides, not that that’s my opinion.
“Although we’ve been afflicted by this riot, I still understand it. When people have enough, this is what happens.”
Rubbish and wheelie bins were set alight by rioters as they were moved through the streets by armoured police.
