Petrol bombs, bricks and fireworks have been thrown at officers after trouble erupted for a second consecutive night in the Castlereagh Street and Templemore Avenue areas.
A PSNI spokesman said crowds of up to 300 people were involved in the rioting.
"A number of officers have been injured and several arrests made," he said.
"Police continue to advise motorists to avoid the area."
The sporadic violence intensified during the night and water cannon was deployed to the scene.
UTV's Sharon O'Neill said the area was "tense" amid stand-offs and a heavy police presence.
She tweeted: "Scuffles as police make arrest in east Belfast".
Earlier a loyalist flag protest was held at Woodstock Link.
It came after First Minister Peter Robinson strongly condemned the street violence in the area, which left 10 police officers injured on Thursday night.
The violence and destruction visited on the PSNI is a disgrace, criminally wrong and cannot be justified.
Mr Robinson said those behind the unrest were playing into the hands of dissident republicans.
"Those responsible are doing a grave disservice to the cause they claim to espouse and are playing into the hands of those dissident groups who would seek to exploit every opportunity to further their terror aims," he said.
Last month, Belfast councillors voted to limit the number of days the Union Flag is flown over City Hall - a decision which the DUP leader has described as "ill-considered and provocative".
"All right-thinking unionists will want to channel their energies into political activity and to support the cause of finding political solutions to the problems that we face," Mr Robinson said.
"In Northern Ireland the ballot box has primacy and is the only vehicle for choosing the people's representatives."
"The real purpose of those who claim to speak for some of the organisers of the protests is fast becoming clear," he added.
"It is a straight-forward politically motivated attempt to undermine the DUP as the voice of unionism despite our attempts to oppose the removal of the Union Flag in Belfast."
On Friday, the Chairman of the Police Federation told UTV he believed the UVF orchestrated violence in the east of the city.
Terry Spence called for harsh jail sentences for anyone involved.
"There's absolutely no doubt this violence was orchestrated by the UVF," he said after the first night of trouble.
More flag protests are planned on Saturday.