A bomb in a car exploded outside the Athens stock exchange on Wednesday, slightly wounding one woman, damaging the building and setting eight vehicles ablaze, Greek police said.
They said a home-made bomb exploded at the same time outside a government building in the northern city of Thessaloniki.
The Athens blast followed a warning received by a Greek newspaper, the police said.
"It was an explosive device. The remains of it were found in a damaged car," said Greek police spokesman Panayiotis Stathis.
The chairman of the Athens stock exchange, Spyros Kapralos, told NET TV: "The building has been seriously damaged."
Earlier, police said the damage to the building was minor.
A Reuters witness said windows of other buildings in the area had been smashed and that a car dealership was among those damaged. The area was cordoned off by police.
Leftist and anarchist guerrilla groups have claimed responsibility for a string of attacks that have struck Greece since the police shooting of a teenager in December sparked the country's worst riots in decades.