It happened on Tuesday 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 has been 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."
A 12-year-old girl has been taken to hospital after she was struck by a missile during minor trouble in Belfast city centre.