Two bullets hit the bus, bursting a tyre, but no one was injured in the incident.
A police official, who asked not to be identified, said the shots were believed to come from the campus.
A group calling itself Popular Action claimed responsibility for the attack in an anonymous phone call to zougla.gr, the news website said.
Police said they found seven shells and two bullet remains from a 7.62 calibre rifle apparently fired from inside a park that forms part of the Athens university campus.
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 fatal police shooting of Alexandros Grigoropoulos on December 6 led to
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 about six per cent.
Leonidas Gotzes, head of international relations at the New York College in Athens, told Al Jazeera earlier this week that there were both internal and external factors in explaining the continuing violence.
"There seems to be a lot more [happening] than meets the eye. There are those that believe that there are forces working both from within and from without," he said.
"Specifically, from within there are opposition groups, anarchists, the radical left, that seem to have jumped on the bandwagon here and are trying to destabilise the government.
Noting that the Costas Karamanlis government has been in power since March 2004, Gotzes said it was elected to cleanse the corruption that has accumulated over 20 years of socialist rule.
"This the government has had a very difficult time bringing about," he said.
"From without, the external factors have to do with primarily the fact that in the last five years the government ... seems to have followed a foreign policy ... that has upset its [European Union] allies.
"Not recognising the independence of Kosovo, preventing Macedonia from entering the Nato alliance and not supporting Cyprus' unification plan. And it is therefore not receiving political support from Europe."