The incident happened this afternoon at Castle Junction.
A man in his 30s was arrested for disorderly behaviour at Donegall Square North.
Police said crowds had gathered in the Donegall Place area and they were monitoring the situation.
Donegall Place was closed to traffic and motorists are advised to avoid the area.
PSNI area commander, Ch Insp Robert Murdie, said: “Police in Belfast city centre are currently monitoring crowds which have gathered in the area.
“Isolated incidents of missiles being thrown have been reported.”
During a stand-off, loyalist youths and republicans youths were kept apart by a line of police officers.
Republicans burned a Union Jack flag.