Police are investigating after a large group of youngsters on bikes rode "dangerously" on one of the busiest main roads into Cardiff.
Video filmed from a car shows the group, estimated to number as many as 100, swerving across lanes, doing wheelies and gesturing at drivers.
A car passenger who shot the film on North Road, Whitchurch, said police needed to act quickly.
South Wales Police has warned young people "of the need to ride safely".
The car passenger, who wants to remain anonymous, said she saw the group on Saturday when they came down the Gabalfa flyover.
"They swerved right up to the car in front of us," she said.
"It was really dangerous.
"None of them were wearing helmets and they were all over the road, sticking their fingers up at people they were going past.
"I really think the police need to do something before someone gets seriously hurt."
Insp Geraint White, of South Wales Police, said: "The city centre road network is extremely busy and not a playground.
"Although many were riding responsibly, a number showed complete disregard for their own safety, as well as putting other road users at risk.
"Whilst we continue to investigate, I would urge all parents to remind their children of the need to ride safely, particularly on the busy roads within Cardiff."
Steve Brooks, national director of Sustrans Cymru, said: "Both drivers and cyclists have a duty to share road space responsibly."
In recent years groups have staged so-called "bikestormz" where they gather on roads in large numbers.
In London last year, hundreds of cyclists took part in one event.