Police have come under attack since Grigoropoulos' killing
Police in Athens have clashed with protesters in a resumption of the violence that flared after last month's killing of a teenager by police.
Hooded youths broke away from a student march against education reforms and threw stones and flares at riot police, who fired tear gas and flash grenades.
The centre-right Greek government has pledged to crack down on the rioting.
Earlier this week a policeman was shot and badly injured in an attack feared linked to the unrest.
The policeman underwent more surgery on Friday.
Prime Minister Costas Karamanlis recently vowed that police would take a harder line against the "catastrophic violence" that swept Greece after the shooting of 15-year-old Alexandros Grigoropoulos on 6 December.
The new deputy interior minister in the prime minister's reshuffled cabinet, Christos Markoyiannakis, has promised a "zero tolerance" approach to crime.
Friday's clashes were the first test of that resolve.