DUBLIN (Reuters) - About 100 homes were evacuated in Northern Ireland's second city Londonderry on Monday after four mortar bombs were found in a van in a residential area of the city, police said.

A 1998 peace deal largely ended more than three decades of violence in the British-controlled province between mainly Catholic Irish nationalists seeking union with Ireland and predominantly Protestant unionists who want to remain part of the United Kingdom.

However, dissident nationalists still stage sporadic gun and bomb attacks, which have intensified in the past four years as frustration with the power-sharing government established under the 1998 deal has grown in parts of the nationalist community.

Militants have targeted government offices in Londonderry in recent months in the run up to the city's 2013 term as UK City of Culture.

Police said they believed the bombs in the van were linked to militant nationalists. They arrested two men in the vehicle and a third in a follow-up operation.

Several roads on the western outskirts of the city were shut while army bomb disposal experts worked at the scene, police said. Part of one of the city's main bridges was also closed.

Local nationalist politician Pat Ramsey told media that police were attacked with stones and at least one petrol bomb as they attempted to evacuate homes.

Discontent with Northern Ireland's power-sharing government in working-class Protestant areas fuelled weeks of rioting in December and January after nationalist councilors voted to end a century-old tradition of flying the British union flag every day over Belfast City Hall.

(Reporting by Ian Graham and Conor Humphries; Editing by Pravin Char)