Unidentified gunman shoots at police bus in Greece

ATHENS, Dec 23 (Reuters) - Two shots were fired at a police bus outside Athens university early on Tuesday, ahead of a planned protest by high school and university students in a third week of anti-government demonstrations in Greece.

No-one was wounded in the shooting which took place around 0300 GMT outside the university, which students have occupied since the Dec. 6 police shooting of a 15-year-old sparked Greece's worst riots in decades.

One of the shots burst a tire aboard the bus, which a police official said was empty at the time except for the driver. Authorities are investigating the incident, which followed a two-day lull in disturbances.

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 in damage and lost business for shopkeepers in the capital.

The unrest has also spread to several other Greek cities.

Athens' streets have been quiet since clashes between police and students on Saturday night but a protest march was scheduled on Tuesday in the central Syntagma square outside parliament.

The fatal police shooting of Alexandros Grigoropoulos 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 six percentage points. Some analysts say months of street protests could force early elections next year. (Reporting by Daniel Flynn; Editing by Jon Boyle)