Belfast, Northern Ireland (CNN) -- Six police officers were injured in clashes in a number of Protestant areas of Northern Ireland overnight, police said Sunday, as an annual period of tensions between Catholics and Protestants begins.
Politicians said the trouble flared as a result of flags being removed by police officers in Ballyclare, County Antrim.
The Police Service of Northern Ireland said the flags were paramilitary and illegal.
Police said the violence started in Ballyclare just before midnight Saturday when a crowd of up to 100 people attacked officers with petrol bombs and other missiles.
Five officers were hurt when a hijacked bus was rammed into their vehicle, while another was hit by masonry, police said.
Police fired plastic bullets and a spokeswoman said "while it is believed people were struck, we have no reports of any injuries at this stage."
A water cannon was used to disperse the rioters and police said the trouble in Ballyclare ended shortly after 2:30 a.m. Sunday, police said.
Police said there were further disturbances in the nearby Ballyduff area of Newtownabbey and in Carrickfergus with a number of vehicles hijacked and burned.
There were shots fired at two different homes in the province, police said, but no one was injured.
And rescue crews dealing with suspicious fires at a third location were attacked by young people wearing balaclavas, police said. They did not report any injuries.
Those incidents all took place in mainly Protestant areas of Northern Ireland, police said.
Meanwhile, homes have been attacked during disorder in Magherafelt in County Londonderry.
Police said they were called to the area around 3 a.m. following disturbances involving 30 to 50 people close to a loyalist bonfire.
Police said a barricade was set up across a road and when officers tried to remove it they were attacked with missiles.
Homes were attacked and police said there were treating the incidents as sectarian.
Calm was restored around 5 a.m. and a 27-year-old man was arrested for riotous behavior, police said.
The violence comes as marching season begins - the annual parades by different Protestant organizations to mark historic battles between rival claimants to the British throne.
The province suffered decades of violence known locally as the Troubles, in which over 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.