Fresh violence has flared in Northern Ireland as loyalists protest over the flying of the union flag.
The PSNI said they are also investigating reports that a number of shots have been fired at police lines on the Newtownards Road.
A 38-year-old man has been arrested on suspicion of attempted murder.
Two other men have also been arrested as part of the ongoing disturbances.
More than 1,000 demonstrators marched on Belfast's city hall earlier in the afternoon, but despite tense scenes the rally passed off without major incident.
Protests and demonstrations continue at the removal of the Union flag from the city hall
There was a heavy police presence, including officers in riot gear with dogs, stationed within city hall itself and on surrounding side streets.
But as the flag-waving crowds dispersed, ugly scenes erupted on the Newtownards Road in the traditionally unionist east of the city.
Police again came under attack from a missile-throwing mob of up to 100 loyalists, who hurled masonry and fireworks at officers.
A section of the Newtownards Road - between Bridgend and Hollywood Arches - was shut down because of the sporadic disorder. There were no immediate reports of any police injuries.
Loyalist violence last 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.
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.
Loyalist Protestors converge on Belfast city hall
The PSNI said they will be seeking further arrests in the coming days in relation to the disorder and have appealed for witnesses.
Conall McDevitt, SDLP policing spokesman, said those organising the demonstrations on social media must be held fully responsible for the resultant violence.
"I appeal for calm today and over the weekend and ask people taking part in protests to reflect upon the violence and ask themselves if they want to be associated with that violence and if that is the best way that they feel they can have their voice heard," he added.
"Nobody is in doubt that some paramilitaries are exploiting these illegal protests to destabilise the situation and ultimately bring violence back on to our streets."
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.
On Thursday 10 police officers were injured during a demonstration in east Belfast. 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.
Police Landrover with the remains of a petrol bomb, thrown by Loyalists in Belfast on Friday
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."
Police in east Belfast were dealing with serious public disorder in the Castlereagh Street and Templemore Avenue areas last night.