A wave of left-wing violence since the police shooting of a 15-year-old in December, which triggered Greece's worst riots in decades, has rocked the fragile conservative government.
"The grenade went off outside the offices of the Network for Political and Social Rights, causing minor damage to the building... and a cafe across the street," a police official who declined to be named said.
Police said they had received no warning and no group had claimed responsibility for the attack yet. The building also houses an immigrants' community centre, which was crowded at the time of the attack, the semi-official state agency ANA reported.
Last month, the left-wing guerrilla group Revolutionary Struggle claimed responsibility for the shooting and serious wounding of a 21-year-old policeman in central Athens.
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