Jenny Sampson was at the scene after two boys, who have not been named, were involved in a crash while riding an electric bike on Snowden Road in the Ely area of the city on Monday night.

She said: "Basically we were all at the scene, the police was just having none of it, they wouldn't let the mums, the dads come up and see their own kids laying on the floor."

"They wouldn't let (the parents) do nothing, it was disgusting how they treated them, and they made them walk home and give them the news in the house, didn't give them any sort of news at the scene, we were there for hours waiting and waiting and they still wouldn't let them through to see if their son was OK."

In the hours after the crash, fireworks and other missiles were thrown at a line of police officers with riot shields who were blocking one end of the street.

A helicopter hovered overhead during the disorder as at least two cars were set alight.

Asked what caused the rioting, Ms Sampson said: "The police, how bad the police treated everybody, you can't treat families like that... With two young kids laying on the floor, and their own family couldn't even get to them, they couldn't even seen their own son's last breaths, it's wrong and it's disgusting."

She added: "They were telling everybody to go away and you can't do that... We were all standing there for about two to three hours before anyone had any information.

"It was really, really bad."

Ms Sampson said she is still unclear what caused the crash but described one of the boys as an "amazing and bubbly guy" who was "always happy, always on the go".

"He liked bikes, he liked scooters... Both of them, that was just their life of liking stuff like that."

South Wales Police said it was investigating the "serious road traffic collision and scenes of violent disorder" which broke out in the residential area.

One person was attacked because rioters thought they were an undercover officer, according to a senior officer at the scene.

Police, including mounted officers on horseback, were seen outside Ely police station in the early hours of Tuesday after suggestions it could be targeted.

Shortly before 3am, rioters moved down Highmead Road in Ely, followed by police officers trying to disperse them.

The rioters continued to throw missiles and set cars alight.

Rubbish and wheelie bins were set alight by rioters as they were moved through the streets by armoured police.

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.

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."

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."

Assistant Chief Constable Mark Travis said: "We received a large number of calls from residents who were understandably frightened by the actions of this large group who were intent on causing crime and disorder.

"The level of violence towards emergency services and the damage to property and vehicles was totally unacceptable.

"Our focus now is to fully investigate the circumstances of the collision and the appalling scenes that followed.

"Arrests have already been made in connection with the disorder and more will follow. We shall be maintaining an enhanced police presence throughout the week and into the weekend."

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.

He said: "They showed nothing but disdain for the community and acted like we didn't deserve to know what happened on our own doorstep."

Mr Urquhart added: "The vast majority of people who were stood in that street were stood there because they wanted to know what would happen next. There were a very small number of people actually doing any sort of violence."

Meanwhile, Cardiff Council's leader has called for calm after the disorder and said it is "really important" a full investigation into the crash can take place.

Councillor Huw Thomas said: "For that to happen we need calm. Any continuation of last night's trouble will only hamper any investigation.

"It's important we all come together, and work together now, to find out exactly what happened. I urge everyone to do all they can to calm the situation."

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".

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.