A 12-year-old girl has been taken to hospital after she was struck by a missile during minor trouble in Belfast city centre.
Crowds gathered in the Donegall Place area and one man was been arrested, police said.
PSNI area commander, Chief Inspector Robert Murdie, said: "Isolated incidents of missiles being thrown have been reported.
"A young girl has been taken to hospital after being struck by a missile thrown from a crowd which had gathered at Castle Junction."
A man in his 30s was arrested for disorderly behaviour at Donegall Square North.
Youths, draped in Tricolours, in Donegall Street set a Union flag alight opposite to where the loyalists, draped in Union flags, had gathered.
Both groups waved their flags as police separated the crowds amid singing and chanting.
A number of loyalist protesters had gathered at Belfast City Hall in what is understood to be the start of a 24-hour vigil.
The minor trouble came after thousands lined the streets of Belfast to enjoy a family day out watching the St Patrick's Day carnival parade.
The Belfast parade began from the city hall at noon and made its way to Custom House Square for a free concert headlined by former Blue singer and Strictly Come Dancing star Simon Webbe.
Stormont, for the first time was among landmark buildings across the world "going green" as part of the Tourism Ireland drive for the celebrations.