Police ask people to stay away from the scene, with some breaking up slabs to throw at officers.
Riot police with dogs have joined emergency services at the scene of a serious road crash after objects were thrown at officers.
Some people wore balaclavas and masks and were seen breaking up paving slabs to hurl at police in Ely, Cardiff.
South Wales Police said they were working to ‘de-escalate’ the situation and urged people to leave the area.
Two police cars were damaged around Wilson Road, where between 100-150 people gathered on Monday evening.
Snowden Road was closed between Wilson Road and Jackson Road and drivers have also been asked to avoid the area.
One police car at the scene had a smashed windscreen and its wing mirrors were broken off.
Pictures on social media showed people and police gathered around a fire on the road.
“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,” police said.
Police tweeted later in the evening: “It is important that people avoid the area so we can effectively deal with this incident.
“Please avoid speculation – we will bring you the latest details as soon as we can – all of our updates must be factual.”
Police posted a fresh update after 23:00 BST which said: “Shortly after 6pm, we received a report of a serious road traffic collision on Snowden Road.
“Police responded to this collision, which had already occurred when officers arrived.
We need to ensure that this evening’s collision is effectively investigated. Officers remain on scene to manage a large scale disorder.
“Again, we urge anyone involved or watching to leave immediately. We understand the concern from local residents and we ask you to stay indoors.”
Cardiff Bus said its service on the road was re-routed and remained so for the rest of the evening.