An explosive device went off outside Greece's labour ministry in central Athens early on Saturday but caused no injuries, Greek police said.
A Greek newspaper received a phone call from an unknown caller that a bomb had been planted at the ministry and had informed the police, who cordoned off the area before the explosion, which caused damage to the building and broken windows, police said.
The caller said a previously unknown guerrilla group was behind the attack, a police official said on condition of anonymity.
Police have launched an investigation into the incident.
Greece has a history of political violence. Small bomb and arson attacks are frequent, and most of them do not cause serious damage.
However, the government said it was worried by the incident.
"The attack is very serious and has to do with a serious crime," government spokesman Pavlos Marinakis told Open TV broadcaster on Saturday.
Hundreds of thousands of people in Australia's Queensland state were without power on Sunday after Alfred, a downgraded tropical cyclone, brought damaging winds and heavy rains, sparking flood warnings.
An Israeli airstrike killed two Palestinians in Rafah in the southern Gaza Strip on Saturday, medical sources said, as mediators pushed ahead with talks to extend a shaky 42-day ceasefire agreed in January between Israel and Hamas.
Toronto Police said early on Saturday they were searching for three male suspects in a shooting that injured at least 12 people at a pub in the Canadian city.
Ex-tropical cyclone Alfred lingered off the south-east Australian coast on Saturday and forecasters said Brisbane is likely to miss the worst of the storm, a relief for millions of residents in the region who have been staying indoors.
South Korean President Yoon Suk Yeol walked out of a detention centre in Seoul on Saturday after prosecutors decided not to appeal a court decision to cancel the impeached leader's arrest warrant on insurrection charges.