Masked men attacked a police car with Molotov cocktails and other objects during a march against the Good Friday peace agreement in Londonderry, police said on Monday, a day before US President Joe Biden’s visit to Belfast.

A Reuters photo shows four young men in Creggan, a predominantly Irish nationalist area, throwing Molotov cocktails at an armored police car that was engulfed in flames on one side.

The crowd soon dispersed, and police said no one was injured.

On Good Friday, April 10, 1998, Republicans who favored Irish reunification and Unionists who wanted to remain in the United Kingdom reached an unlikely peace deal after tense negotiations involving London, Dublin and Washington.

The agreement ended three decades of violence between unionists, mainly Protestants, and republicans, mainly Catholics, with the involvement of the British army.

However, sporadic violence still occurs from small groups opposed to peace.

Late last month, Britain’s MI5 intelligence agency raised the domestic terrorism threat level in Northern Ireland to “severe,” meaning an attack is highly likely.

Alarm levels were raised after an off-duty police officer was seriously injured in an attack by the New IRA, one of the smaller dissident militant groups.

Biden is due to arrive in Belfast on Tuesday and speak at the University of Belfast on Wednesday before traveling to Ireland for a further three days.