Loyalists are believed to have been responsible for a petrol bomb attack close to Donegall Pass police station in south Belfast.
Rival crowds gathered in south Belfast
Five devices were thrown at a car park where officers leave their cars. No-one was injured.
Earlier loyalist and nationalist youths clashed in the Cromac Street area.
They threw stones, bottles, paint and other missiles. Police and community workers spent most of the evening trying to keep the two sides apart.
About 100 people gathered in lower Donegall Pass area and a second group assembled in the lower Ormeau Road area.
Motorists were asked to avoid the area during the trouble.
Sinn Fein has claimed the trouble was prompted by a row over GAA flags in the lower Ormeau area.
The party's assembly member, Alex Maskey, said it was a setback for community relations.
"It appears that the system for dealing with these issues along the interface has broken down," he said.
"If people had asked what these flags were for, then I would suspect that we wouldn't be in this situation."