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.
Qatar's prime minister said on Sunday that efforts to reach a new ceasefire in Gaza have made some progress but an agreement between Israel and Hamas to end the war remains elusive.
A huge blast most likely caused by the explosion of chemical materials killed at least 18 people and injured more than 700 on Saturday at Iran's biggest port, Bandar Abbas, Iranian state media reported.
The UN Refugee Agency (UNHCR) and the UN Office for the Coordination of Humanitarian Affairs (OCHA) have sounded the alarm over severe funding shortfalls that are hindering life-saving humanitarian aid in countries including Nigeria, Burundi, and Colombia.
A number of people were killed and multiple others were injured in Vancouver after a vehicle drove into a crowd at a Filipino street festival in the western Canadian city, police said on Saturday.