Egyptian protesters set cars alight and threw stones at military police in Cairo early on Friday, witnesses and a security source said, after rumours spread that an activist had been detained at a sit-in and badly beaten.
Police fired in the air shortly after dawn to try to disperse around 300 demonstrators, who were angered by images posted online that appeared to show the activist badly beaten after his arrest, the witnesses said.
"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."
There was no immediate reaction from security officials.
However, the security source said several people were injured in the clashes.
The activist, identified by others as Abboudi Ibrahim, was arrested as he left the sit-in outside the cabinet office in central Cairo.
A video circulating on Facebook showed a young man identified as Ibrahim being supported by a crowd, has face badly bruised and his eyes swollen and shut.
Protesters have occupied an area outside the cabinet office since late November to demand the resignation of the military-backed government.
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