Nine officers have been injured in east Belfast after a crowd of up to 300 people hurled fireworks, bricks and petrol bombs during "a sustained attack".
A spokesman for the Police Service of Northern Ireland (PSNI) said 18 arrests were made after the public disorder in the Castlereagh Street and Templemore Avenue areas overnight.
Some 30 petrol bombs were thrown at police officers as they dealt with the fresh riots on Friday night.
A range of other missiles including ball bearings, fireworks and masonry were also thrown.
Three attempted carjackings were reported in the Beersbridge Road area and water canon deployed, but not used.
There were also protests in the Robbs Road area of Dundonald, near Ulster Hospital, and in the O'Neill Road area of Newtonabbey, where there was reportedly an attempt to hijack a bus, plus a raid on a commercial premises by masked men.
The latest violence comes after 10 police officers were injured on Thursday during a demonstration against the decision to reduce the number of days the Union flag is flown from City Hall.
Sky's Ireland Correspondent Vicki Hawthorne said: "The latest disorder has been relatively minor compared to other nights.
"There had been an earlier peaceful flag protest which initially dispersed without incident.
"People in the area have said the police have been too heavy handed in how they have dealt with protesters, and that, they say, has sparked trouble."
Stormont First Minister Peter Robinson described the violence against the police as "a disgrace".
He said: "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 added: "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.
"In Northern Ireland the ballot box has primacy and is the only vehicle for choosing the people's representatives."
Officers are appealing for witnesses or anyone with information to contact their local police station on 0845 600 800, or Crimestoppers anonymously on 0800 555 111.