Water cannon were also deployed as trouble flared when loyalists returned from a city-centre demonstration against Belfast City Council's decision to limit the number of days the Union flag is flown from the city hall.
Riot police attempted to separate opposing factions amid a hail of bricks and fireworks at the Albertbridge Road near the nationalist Short Strand.
The trouble began at about 2.30pm yesterday when loyalists and nationalists clashed at the sectarian interface in the east of the city.
Police fired baton rounds as they came under sustained attack when moving in to separate the groups. A car was later set on fire at the junction of Castlereagh Street and Templemore Avenue.
UUP leader Mike Nesbitt appealed for calm to allow talks to make progress. "Street violence from so-called unionists, no matter what age, advances nothing but the cause of Irish nationalism," he said. "It is high time those involved realise they are destroying the very cause they hope to promote."
Earlier, nearly 1000 people had gathered at Belfast City Hall to protest.
The demonstrations against Belfast City Council's decision to fly the Union flag from the city hall only on designated days such as royal birthdays has brought many parts of the city, and other parts of the north, to a standstill.
More than 70 officers have been injured and over 100 arrests made during weeks of sporadic trouble, according to the Police Service of Northern Ireland.
Senior politicians from Belfast, Dublin and London are due to meet this week to discuss the protests after more than 40 days of roadblocks and sporadic violence by loyalists have failed to produce a solution.
Stormont First Minister Peter Robinson and Deputy First Minister Martin McGuinness will join Northern Ireland Secretary Theresa Villiers and Ireland Tanaiste (Deputy Prime Minister) Eamon Gilmore.
Gilmore said: "This violence is being orchestrated and those behind it are known criminals, intent on creating chaos.
"This has nothing to do with real issues around flags and identity in a shared society, which are the subject of intensive political discussions at present."
The council, which now has a nationalist majority, voted to fly the flag at Belfast City Hall on 17 designated days, as opposed to every day of the year.
The majority of the street demonstrations have passed without incident, but some have resulted in serious rioting.
Businesses in Belfast's city centre have struggled to cope, with many reporting lost trade, since the demonstrations began.
A doctor was prevented from attending a terminally ill cancer patient because of loyalist road blocks in south Belfast, it was revealed.
The GP was travelling to a home call when he was stopped twice by crowds of demonstrators who blocked the road. Police asked them to move but they refused, nationalist SDLP MLA Conall McDevitt said.
He said: "These are depraved acts which immediately dismiss any claim of a protest being peaceful."
The doctor had to wait until the blockade was lifted.
On Friday night, four police officers were injured during Union flag protests in Northern Ireland.
Serious violence has also taken place outside Belfast, in Newtownabbey and Carrickfergus in County Antrim. Officers fired five plastic bullets as rioters threw more than 30 petrol bombs.
Fireworks, petrol bombs and rocks were used to attack police in Newtownabbey, a bus was set alight near the Rathcoole estate, and a crowd of more than 100 loyalists threw missiles at police in Carrickfergus.
Meanwhile, more than 100 protesters gathered outside the City Chambers in Glasgow on Friday to show solidarity with the unionist cause in Northern Ireland.
Some waved the union flag and sang the national anthem in protest at Belfast City Council's decision to stop flying the flag over City Hall.
A unionist forum chaired by Democratic Unionist Party leader Robinson and UUP leader Nesbitt met last week to discuss ways of empowering loyalist working-class communities. A peace rally is to be held outside Belfast City Hall today.