Police have said they came under "heavy gunfire" and arrested 31 protesters in the latest flare-up of violence in Ferguson, Missouri.
Police fired tear gas and stun grenades as peaceful protests descended once again into chaos after nightfall.
The St Louis town has been beset by unrest since the August 9 shooting death of Michael Brown (18), a black teenager who was killed by a white policeman.
Officers in riot gear, supported by an armoured SWAT vehicle and a helicopter, repeatedly ordered the crowd of about 100 to disperse. Reports from the scene suggested bottles were thrown and live ammunition was fired.
The shots appear to have prompted police to fire a volley of tear gas just after 11pm (5am Irish time), sending the crowd fleeing.
The crowd that gathered near where Mr Brown was shot dead by a local policeman was smaller than the one that clashed with officers on Sunday.
Mr Brown's death has sparked daily protests and nightly clashes with police in the St Louis suburb and prompted the deployment of National Guard reservists. But the National Guard appeared to keep their distance.
Coroner's sketch shows Mr Brown was shot several times
Pathologists hired by his family previously revealed he had been shot at least six times, including twice in the head.
Mr Brown was shot in broad daylight on a residential street last Saturday by Darren Wilson (28), a white police officer.
Different versions of the shooting have emerged, with police sources saying there was a scuffle during which Mr Brown tried to seize the policeman's weapon.
Witnesses have alleged the teenager had his hands up and was not resisting when he was shot. The Washington Post said traces of marijuana were found in his system.
A total of three autopsies have been requested - by local authorities, the family and the Justice Department.
Officials have said a grand jury could hear evidence in the case as early as Wednesday.
President Barack Obama has called for calm, saying there was no excuse for police to use "excessive force". He has also urged "limited" use of the National Guard.
National Guard troops arrived in Ferguson earlier in the day but remained out of sight at a police command centre.
Michael Calhoun is a reporter with CBS Radio in St Louis. He told Newstalk Breakfast what has been happening there this morning.
Originally published 06:30
