Belfast, Northern Ireland (CNN) -- At least 47 police officers were injured overnight trying to keep Catholics and Protestants apart in troubled Belfast, Northern Ireland, police said Monday.
Four of them had to be sent to hospitals with injuries to the head, legs and chest, the Police Service of Northern Ireland said.
Protestant demonstrators were opposing a scheduled parade by a Catholic marching band and its supporters, and police were on the scene to maintain order.
Police were attacked with "fireworks, petrol bombs, masonry and lasers," and responded with water cannons, they said.
It was the largest number of police officers hurt in a single disturbance since the summer of 2010, when more than 80 officers were injured in rioting in the Belfast neighborhood of Ardoyne, a frequent flashpoint between Catholics and Protestants.
The violence came as Protestants who want Northern Ireland to remain part of the United Kingdom clashed with Catholics who want the province to become part of the Republic of Ireland.
Police arrested a 17-year-old male on suspicion of "riotous behavior" and are expecting more arrests, they said. He is helping police with their inquiries, they said without naming him.
The violence took place near Ardoyne.
On Sunday, a Protestant establishment, the Belfast Grand Orange Lodge building, was attacked. Tom Haire of the lodge blamed the aggression on "those whose hatred and intolerance of all things Protestant and British is manifested in the campaign against traditional parades."
Tension, which often rises over the summer, has been building since a Protestant band played outside a Catholic church on July 12. More than 20 police officers were injured that day, the PSNI said.