Egyptian protesters set cars alight and threw stones at military police in Cairo on Friday, witnesses said, after rumours spread that an activist had been detained at a sit-in outside the cabinet office and badly beaten.

Police fired in the air after dawn to try to disperse around 300 demonstrators who were angered when images were posted online of the man - named as Abboudi Ibrahim - being supported by a crowd, his face badly bruised and eyes swollen and shut.

The rumour is they beat him up badly and he is in hospital, said a doctor at a field hospital set up to treat injured demonstrators. This led people to go down to protest.

One security source said several people were injured in the clashes, but there was no official comment on the violence.

Video circulating online showed unidentified men hurling rocks down on rioting demonstrators from rooftops.

A government building near the cabinet office was on fire and the clashes continued through the morning, the state news agency MENA reported.

Protesters have occupied an area outside the cabinet office since late November, forcing the government to hold its meetings elsewhere.

The military police are trying to break up the sit-in in front of the cabinet and unsurprisingly using force against peaceful protesters, said 24-year-old activist Mohamed Aref.

MENA said activists were urging others to go to Cairo's Tahrir Square to hold fresh protests.

The sit-in is what remains of far bigger protests that left dozens of people dead and overshadowed the build-up to Egypt's first parliamentary election since the overthrow of President Hosni Mubarak in February.

The six-week election has been mostly peaceful since it began on November 28. A big turnout has taken some of the steam out of street protests aimed at pressuring the military to hand power immediately to civilians.