Petrol bombs were hurled at police during a republican parade in Derry.
Flames ignited around heavily armoured vehicles bearing officers patrolling the illegal demonstration by dissident republicans.
Eyewitnesses put the total of fire bombs at dozens but senior officers counted seven. There were no reports of injuries or arrests made.
Heavy masonry was used to smash the windows of PSNI vehicles in the nationalist Creggan estate, an eyewitness said.
Sporadic trouble continued for around two hours on Monday.
Republicans mark the anniversary of the 1916 Easter Rising every year.
Police warned demonstrators that it was an illegal parade and said they had failed to discuss the event with them beforehand.
Chief inspector Ivor Martin said: "It is, therefore, regrettable that in raising those tensions a small number of people saw fit to attack police officers with petrol bombs in what was clearly a premeditated attack.
"Police will review all video footage collected today and will robustly pursue all possible inquiries to identify those involved and bring them before the courts."
Foyle Democratic Unionist Assembly member Gary Middleton said the scenes were disgraceful.
"People in paramilitary-style uniform is wrong and the continued glorification of terrorism is abhorrent.
"Nobody has a desire to see the violence on display today, except for barbaric thugs intent on endangering life and property.
"The PSNI were there to protect the community - people who live in fear at displays and parades such as this one today.
"The police have to be praised for the work they continue to do this weekend, as they uphold the law and thoroughly deal with illegal parades and displays of such a disturbing nature."
He said young people hurled petrol bombs.
"I hope the actions of all involved are investigated and those responsible feel the full weight of the law.
"The over-riding desire of many is to see events pass off peacefully and without any paramilitary trappings, unfortunately a minority in our communities are intent on creating destruction and fear.
"Again the demands of respect and equality from republicans ring hollow when there is little shown to those who uphold the law and work to keep communities safe.
"Those involved should not believe they are above the law."