A 38-year-old man was arrested yesterday on suspicion of attempted murder, while two others were also detained amid ongoing street disturbances.
Last night officers in east Belfast came under sustained attack from a mob of around 100 loyalists throwing petrol bombs, fireworks, bricks and other masonry, the PSNI said.
A spokesman said officers dealt with disorder in the Newtownards Road, Albertbridge Road and Castlereagh Street and Templemore Avenue areas of the city, using water cannon and firing three AEP rounds (rubber and plastic rounds).
One officer was injured and a number of arrests were made, the PSNI said.
More than 1,000 demonstrators marched on Belfast's City Hall yesterday, but despite tense scenes and some scuffles the rally passed off without major incident.
There was a heavy police presence, including officers in riot gear with dogs stationed within the historic civic building itself and on surrounding side streets. But as the flag-waving crowds dispersed, ugly scenes flared again - for the third night in a row - on the Newtownards Road in the traditionally unionist east of the city.
A section of the road was shut down because of the sporadic disorder.
A PSNI spokesman said they were investigating reports from their own officers that a number of shots were fired at police lines.
Conall McDevitt, policing spokesman for the nationalist SDLP, said the firing of shots at police officers ended any claim to legitimacy by protest organisers.
"Whatever grievance some people may have had, it is totally lost when they allow people to use these protests as cover for attempted murder," said the South Belfast MLA.
"There is only one response possible - and that's a firm policing response against everyone involved in illegal protests and anyone seeking to organise or encourage illegal or violent demonstrations."
Mr McDevitt urged unionist leaders to publicly reject the protests "before someone is killed or seriously injured".
Loyalist violence on Friday night saw 18 people arrested and nine police officers injured. More than 30 petrol bombs, along with fireworks, ball bearings and masonry were hurled at officers during a sustained attack in the east of the city. Up to 300 people were involved in the disturbances.
None of the police injuries are life threatening, however one female officer required medical treatment at the scene by an ambulance.
Three attempted vehicle hijackings were made in the Beersbridge Road area while a business premises on the O'Neill Road was broken into, damaged and money stolen from it.
The PSNI said it would be seeking further arrests in the coming days in relation to the disorder and have appealed for witnesses.
On Thursday 10 police officers were injured during a demonstration in east Belfast. Stormont First Minister Peter Robinson yesterday said violence against the police was a "disgrace" and those behind days of unrest were playing into the hands of dissident republicans.
Street protests have been going on for more than a month now against the decision to reduce the number of days the Union Flag is flown from Belfast City Hall. There have also been death threats to politicians.
Mr Robinson said: "The violence and destruction visited on the PSNI is a disgrace, criminally wrong and cannot be justified.
"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 some demonstrators were employing language borrowed from the republican handbook to demonise police and undermine the DUP through a call for direct rule from London.
"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."
Ireland's Tanaiste (deputy prime minister) and Foreign Affairs Minister Eamon Gilmore condemned the latest attacks on police officers.
"These attacks over the past three days are not the mark of legitimate protest but are the actions of a small group who want to bring Northern Ireland back to a darker past," he said.
Mr Gilmore said the Irish Government fully supports the efforts of and will remain in close contact with elected leaders in Northern Ireland to resolve the flags issue.