Ballistics tests show two military-type automatic rifles were used in an attack on a riot-police bus in Athens, authorities said Wednesday. The discovery raised fears of renewed violence by urban guerrilla groups that have targeted police in the past.

None of the 19 policemen on board was hurt in Tuesday's pre-dawn attack, in which at least seven shots were fired at the bus as it passed a university campus.

Authorities were also investigating a claim of responsibility for the attack made in an anonymous call to a news Web site. The caller reportedly said a previously unknown group calling itself Popular Action carried out the shooting.

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Protests and attacks on the police have continued since the Dec. 6 police shooting of a teenager, which sparked the worst rioting Greece has seen in decades.

Violence has eased after two weeks of rioting. On Christmas Eve, about 700 mainly young protesters marched through Athens' main shopping district to demand the release of those arrested during the rioting. The demonstrators sang Christmas carols with protest lyrics, and scattered fliers calling for the release of those arrested. No violence was reported.

A public prosecutor who handles terrorism offenses was heading an investigation into the shooting.

But police spokesman Panayiotis Stathis said it was too soon to call the shooting a terrorist attack.

At least six serious attacks have been carried out by little-known radical groups in the past five years, including two bombings and the murder of a policeman by gunmen who stole his automatic weapon.

Most of the serious attacks were claimed by a group called Revolutionary Struggle.