The shooting occurred close to a university campus
Bullets fired by an unidentified attacker have hit a riot police bus in the Greek capital Athens, causing no injuries but alarming the authorities.
Two shots hit the vehicle, bursting a tyre, in an attack mounted near a university campus.
Hundreds of demonstrators, including many students, later held a peaceful anti-government march through the city.
The march came after more than two weeks of unrest over the killing of a teenager by a policeman.
Athens has been relatively quiet in recent days after the worst riots in decades destroyed hundreds of businesses in the capital and in other Greek cities.
The policeman accused of shooting Alexandros Grigoropoulos, aged 15, has been arrested and charged with murder. He insists the teenager was killed by a ricochet.
In Tuesday's flashpoint, an unidentified attacker opened fire on the bus, which was carrying 19 police officers, when it stopped at traffic lights outside a campus.
Protests in Athens have continued for more than two weeks
A police spokesman, who did not wish to be identified, was quoted by Reuters news agency as saying the shots were believed to have come from the campus and were fired from a military weapon.
Police spokesman Panagiotis Stathis told Mega TV that seven bullet cases had been collected from the site. He said the incident was "worrying".
Authorities are investigating a call from a previously unknown group called Public Action to a popular Greek news website claiming responsibility.
While the march passed off peacefully, a group of demonstrators overturned an empty police car.
Marchers chanted "cops, pigs, murderers" as they demanded the resignation of the conservative government, which has a one-seat majority in parliament.
The demonstrators set fire to a large paper pig's head sporting a policeman's cap and dumped it at the feet of riot police guarding the parliamentary building.