A number of masked people attacked police 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 vehicles, which was covered in flames on one side. Police said nobody was injured and appealed for calm.

“Our officers have come under attack in Creggan with petrol bombs and other objects thrown at their vehicle while in attendance at an un-notified Easter parade. No injuries have been reported at this time,” police said in a statement.

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.