A number of masked people attacked a police vehicle with petrol bombs and other objects at a parade opposing the Good Friday peace accord in Londonderry, police said on Monday, a day before U.S. President Joe Biden visits Belfast.
A Reuters photograph showed four young people in the mainly Irish nationalist area of Creggan throwing petrol bombs at an armoured police vehicle, which was covered in flames on one side.
The crowd dispersed shortly afterwards and police said nobody was injured.
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 peace.
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 at 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 travelling to Ireland for a further three days.
