PORTADOWN, Northern Ireland (Reuters) -Rioters attacked police with petrol bombs, rocks and fireworks in the Northern Irish town of Portadown on Thursday, a Reuters witness said, as a fourth night of anti-immigrant violence moved to a different part of the British-run province.

Hundreds of masked rioters attacked police and set homes and cars on fire in the Northern Irish town of Ballymena on Tuesday, in the second successive night of disorder that followed a protest over an alleged sexual assault in the town.

Spurred by false racist rumors, a fourth consecutive night of violence has plunged Northern Ireland into crisis, as anti-immigrant rioters once again attacked police, throwing bricks and masonry. The unrest, triggered by the arrest of two local teenage boys in connection with an alleged sexual assault earlier this week, continued on Thursday night.