Belfast , Northern Ireland ( CNN ) -- Sectarian violence flared up Saturday in Londonderry as Protestants continued their annual marching season in this predominantly Catholic city in Northern Ireland . Violence erupted earlier in the day in the city -- also known as Derry to some residents -- when masked youths attacked police vehicles with petrol bombs and a pipe bomb , setting fire to and hijacking a number of vehicles . The clashes followed the annual Apprentice Boys ' parade , when about 10,000 Protestants marched in remembrance of the 1689 anniversary of the siege of Derry by England 's Catholic ruler James II . Police said protesters hurled several dozen petrol bombs early Saturday and a mail van was later hijacked and set on fire as the main parade started . The Police Service of Northern Ireland warned members of the public to avoid the area . Three people have been charged in connection with the disorder , with several more arrested , police said . The area was later reported quiet , with local sources describing the earlier violence as `` sustained rioting during a short period of time . '' However , there was further sporadic violence later in the evening . Derry 's hard-line Catholic , republican-nationalist groups opposed to the Irish peace process have reacted violently in previous years to these annual Protestant marches , seeing them as provocations . Protestant groups claim they have the right to follow their centuries-old tradition . There was rioting earlier in the summer with some of the worst violence in the Ardoyne area of north Belfast following a parade by the main Protestant marching organisation , the Orange Order . The province suffered decades of violence known locally as the Troubles , in which more than 3,500 people were killed before a 1998 peace accord known as the Good Friday Agreement . The Troubles pitted mostly Catholic republicans , who want the province to become part of the Republic of Ireland , against pro-British loyalists , who are mostly Protestant . This year has been one of the most violent since the Good Friday Agreement .
Protestants riot in Londonderry , a predominantly Catholic town . Masked youths attack police vehicles . Sporadic violence continues into the evening .