(Adds police spokesman, details paragraphs 3-5)

By Daniel Flynn and Lefteris Papadimas

ATHENS, Dec 23 (Reuters) - Hundreds of anarchists chanting "cops, pigs, murderers" marched through Athens on Tuesday hours after a gunman opened fire at a riot-police bus in a third week of anti-government protests since police shot dead a teenager.

An unidentified gunman shot at the bus carrying 19 officers when it stopped at traffic lights outside a university campus in eastern Athens at around 5 a.m.

Two bullets hit the bus, bursting a tyre, but no one was injured in the incident, which is being investigated by counter-terrorism police.

It came after a two-day lull in Greece's worst riots in decades, sparked by the shooting of 15-year-old Alexandros Grigoropoulos.

"It's worrying us," police spokesman Panagiotis Stathis told Mega TV. "We collected seven bullet cases from the spot."

A police official, who asked not to be identified, said the shots were believed to come from the campus and were fired from a military weapon. Authorities are investigating a call from a previously unknown group called Public Action to a popular Greek news website claiming responsibility.

Police are forbidden by law from entering the university without permission. It has become the epicentre of disturbances which have caused hundreds of millions of euros (dollars) in damage and lost business for shopkeepers in the capital.

Some 500 anarchist students, including dozens who are occupying the Athens law school, marched through central Athens, many of them chanting "cops, pigs, murderers" and waving red and black flags. They burnt an effigy of a pig's head wearing a police hat outside parliament.

Unlike demonstrations during the previous two weeks, the protests remained largely peaceful. The only incident reported was when a gang of youths who broke away from an earlier student march overturned an empty police car in central Athens.

The fatal police shooting of Grigoropoulos on Dec. 6 unleashed widespread discontent at high youth unemployment, government scandals, right-wing reforms and an economic slowdown due to the global crisis.

The violence has shaken a conservative government that has a fragile one-seat majority and trails the opposition in opinion polls by around 6 percentage points. Some analysts say months of street protests could force early elections next year.

(Reporting by Daniel Flynn; Editing by Michael Roddy)

AlertNet news is provided by