Petrol bombs and other objects were thrown at Northern Ireland police by a number of masked people at a parade opposing the Good Friday peace accord, authorities said Monday.
The attack on a police vehicle in the city of Londonderry comes a day before U.S. President Joe Biden's visit to Belfast.
A Reuters photograph showed four young people in the mainly Irish nationalist area of Creggan throwing petrol bombs at an armored police vehicle, which was covered in flames on one side.
The crowd dispersed shortly afterward, and police said nobody was injured.
"What we saw develop this afternoon in Creggan was incredibly disheartening," said Derry City & Strabane Area Chief Superintendent Commander Nigel Goddard.
"This was a senseless and reckless attack on our officers who were in attendance in the area in order to comply with our legal duties," he added.
He said police have deployed evidence-gathering resources, and obtained footage which will now be reviewed as part of an investigation into potential offenses under the Terrorism Act 2000.
Monday marked the 25th anniversary of the signing of the Good Friday Agreement that largely ended three decades of sectarian bloodshed in Northern Ireland. There is still some sporadic violence by small groups opposed to the accord.
Britain's MI5 intelligence agency late last month increased the threat level in Northern Ireland from domestic terrorism to severe – meaning an attack is highly likely.
It has been mostly in the second highest category since its introduction in 2010 and was raised after an off-duty police officer was left seriously injured following a gun attack by the new IRA, one of the small dissident militant groups.
Biden is due to arrive in Belfast on Tuesday and make an address at a Belfast university on Wednesday before traveling to Ireland for a further three days.
