A letter bomb blew up in a police station in Turin yesterday, slightly injuring a policewoman, and a second explosive device was intercepted in a residence for immigrants, also in northern Italy, police said.

Investigators said a note inside the package which did not explode suggested it had been sent by anarchist militants to the immigration centre in the city of Modena.

The device which did go off was wrapped in an envelope filled with bolts.

"The agent realised something was odd, quickly tossed the envelope away from her and it exploded with a burst of flame that blackened the wall," a Turin police official said.

Last week police arrested a group of suspected anarchist militants whom they accuse of similar explosive attacks in several Italian cities.

Small bombs exploded in Milan, Turin and other cities in March, damaging cars and buildings but causing no casualties. They had mostly been placed in rubbish bins and containers, going off at night.