A coach full of English Defence League supporters was pelted with missiles after it broke down in east London.
The coach was carrying 44 EDL members when it stopped in Mile End Road, Tower Hamlets.
About 100 Asian teenagers then pelted it with bricks and stones, according to a BBC reporter at the scene.
Police arrested all 44 EDL supporters, who were travelling from a protest in Aldgate earlier on Saturday. A double decker bus was used to evacuate them.
Police said there had already been one altercation with local youths after the vehicle stopped in Whitechapel Road and some passengers got off the coach.
They got back on board and the coach pulled away – but it later suffered a failure and ground to a halt.
BBC reporter Paraic O’ Brien, who was on the scene, said nearly 100 local teenagers then attacked it with missiles.
He said the police were on the scene extremely quickly.
The reporter said within a short space of time there were a number of riot vans and 200 police officers in the vicinity.
O’Brien said: “It was extremely tense and if that number of officers had not arrived it could have gone the other way and become a major incident.”
The police moved the EDL supporters onto a double decker bus, and escorted it east.
Protesters and police The earlier demonstration was rowdy but major disorder was avoided
But a group of youths subsequently sat down in the middle of Mile End Road, blocking the bus and forcing it to stop.
At this point a large number of Asian men began arriving from a nearby estate.
The reporter said by then the situation had become very scary.
The police charged the youths and scuffles broke out.
Another group standing on a footbridge over the road threw bricks at the bus.
Police managed to clear the road and the bus left the area.