The level of violence towards emergency services and damage caused by riots that broke out in Cardiff after two teenage boys died in a crash has been described by police as “totally unacceptable”.

Tensions reached breaking point after officers were called to a collision, in Snowden Road, Ely, shortly after 6pm on Monday.

South Wales Police said the collision had already occurred when officers arrived, and they remained on the scene to manage “large-scale disorder” until the early hours of Tuesday morning.

The force said arrests have been made but did not say exactly how many.

Specially trained public order officers were deployed, including officers from neighbouring police forces, as several vehicles were set alight, property was damaged and officers were injured.

Assistant Chief Constable Mark Travis said: “First and foremost, our thoughts are with families of the two boys who have died following the collision in Ely and with those affected by the disorder which followed.

“These are scenes we do not expect to see in our communities, particularly a close-knit community such as Ely.

“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.

“We would like to thank the public for their support and appeal to anyone with information or video footage to come forward and provide that to us.”

Scenes being livestreamed on YouTube showed young people throwing fireworks and other missiles at a line of police officers with riot shields who were blocking one end of the street.

Shortly before midnight a car was set on fire and burned fiercely, while a second vehicle was overturned and also torched.

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.

Alun Michael, South Wales Police and Crime Commissioner, 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 that you’ve referred to 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 on social media nowadays, and that events can get out of hand.”

Mr Michael said people will be “brought to account” for the violence.

“Obviously very frightening for local people. And there were a lot of calls to the police expressing concern about what was going on,” he said.

Mr Michael added: “Ely is an area which has a lot of deprivation, but it’s also a very warm community.

I urge everyone to do all they can to calm the situation

“I think what we have is an incident that involved some people and a proper investigation is needed to establish exactly what happened.”

Downing Street said the scenes were “appalling and completely unacceptable”.

The Prime Minister’s official spokesman said: “The reports of disorder, violence, arson and specifically attacks on police officers are appalling and unacceptable.

“We remain grateful to the emergency services and first responders for all their support to the community and restoring order.”

Cardiff Council’s leader called for calm and said it is “really important” a full investigation into the crash can take place immediately.

Councillor Huw Thomas said: “My thoughts and deepest sympathies this morning are with the families of the young people who died in the road collision yesterday. This is a tragic event and they, and we, need to understand what happened and why.

“The best way we can achieve this is by letting the investigation begin as soon as possible. Any further trouble will only delay that investigation.

“I know the majority of people in Ely are deeply worried and concerned about the scenes last night.

“It’s really important that a full investigation into what happened, and into how the road collision occurred, can take place immediately.

“For that to happen we need calm. Any continuation of last night’s trouble will only hamper any investigation.

There must now be a full investigation to understand how this tragic incident led to the violence that followed to ensure peace is restored and that this never happens again

“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.”

Welsh Conservative Leader Andrew RT Davies said there is “no excuse” for the disorder that unfolded and said it was “deeply concerning”.

“There is simply no excuse for this kind of disorder in our communities and it is unacceptable that people have been made to feel unsafe in their own homes.

“My deepest condolences go out to the families of the two boys who lost their lives last night. My thoughts are also with the injured police officers, and I wish them a speedy recovery.

“I have submitted an emergency question in the Senedd today, to seek answers from the Welsh Government on what discussions they are having with South Wales Police regarding this concerning situation,” he said.

Plaid Cymru MS for South Wales Central Heledd Fychan said: “There must now be a full investigation to understand how this tragic incident led to the violence that followed to ensure peace is restored and that this never happens again.”