Swansea residents are being told to stay indoors after violent disorder broke out in the Mayhill area of the city.
A number of officers attended the scene, where several cars have been set alight and crowds have gathered on the streets.
Footage shared on social media also shows people throwing items at a police van.
South Wales Police has said it believes the disturbance started following a vigil for a local resident who had recently died.
Superintendent Tim Morgan said: "What is believed to have started as a vigil to a recently deceased local resident has escalated into violence, criminal damage and a wanton disregard for public safety."We have a number of officers at the scene working to de-escalate the situation as quickly as possible and allow other emergency service colleagues to attend to calls in the area."The incident is still on-going and we urge anyone involved to leave the scene immediately."Those involved can expect to face robust action. Post incident investigations will also aim to identify all those concerned."
Officers are urging anyone with information or footage of the incident to contact them.
The leader of Swansea Council has condemned the violence and the "disgusting behaviour by yobs".
In a Facebook post, Cllr Rob Stewart said: "Absolutely disgusting behaviour by yobs in Mayhill Waunwen tonight.
"This is completely unacceptable and inexcusable behaviour. Our thoughts are with the residents who have had to put up with this utterly despicable behaviour in our community.
"Our officers have linked in with the police who have been on the scene for sometime.
"We will support the police to identify, pursue and prosecute those that have organised and participated in this criminal activity."
A clear-up organised by the local community is set to take place on Waun Wen Road on Friday morning.
Welsh Conservative MS for South Wales West Tom Giffard described the scenes as "disturbing".
In a social media post, he said: "Disturbing scenes from Mayhill on social media this evening.
"Police are in attendance and are asking the public to stay away from the area. Hoping this gets sorted as quickly as possible."