Unverified posts on social media later speculated that the collision had been caused by a police chase.
Police urged people “not to speculate” online, and said the crash had already happened before officers arrived.
In an initial statement on Twitter, South Wales Police Cardiff said: “Emergency services are at the scene of a serious road traffic collision on Snowden Road, Ely.
“The road is closed between Wilson Road and Jackson Road. Please avoid the area and take an alternative route.”
In a later update, the force said there were “now 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,” police said.
“Emergency services remain at the scene. 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.”