A small, homemade bomb exploded Tuesday outside the office of former Greek Prime Minister Costas Simitis, causing a fire in the building, police said. No one was hurt.

No group immediately claimed responsibility for the attack, which occurred on the fifth floor of an apartment block in central Athens, and police said they had received no warning call about it. But arson attacks by anarchist groups are common in the Greek capital.

Those attacks have typically used gas canisters, and police said the latest bomb was made with such devices.

Greek police have stepped up patrols ahead of a general election, expected at the end of the month or in early May, following a spike in violent crime blamed on the country's painful recession.

Simitis, a Socialist, was premier from 1996 to 2004. His office was targeted in an arson attack in 2010. Back then, a group calling itself Action for Revolutionary Liberation claimed responsibility, though it was unclear whether authorities considered the claim to be genuine.

Statements from the police and Fire Service said 27 firefighters were dispatched to the scene of the blaze Tuesday.

Government spokesman Pantelis Kapsis denounced the attack.

"Those who try to wreck the political atmosphere ahead of the general elections will fail," Kapsis said. "Democracy cannot be terrorized."