Athens  - A group of 20 self-styled anarchists attacked riot police and burned several cars in Athens overnight, causing damage but no injuries, authorities said Saturday.

The 20 hooded youths hurled homemade Molotov cocktails at riot police on guard outside the old offices of the main opposition Socialist party in the area of Exarchia. Riot police retaliated by spraying tear gas.

The youths escaped through the narrow streets of Exarchia, burning two cars and an ATM machine.

Earlier this week, an explosion outside the Athens Stock Exchange injured one person, caused minor damage to the building and destroyed at least eight cars parked nearby.

At about the same time, another explosion occurred in front of a ministry building in the northern port city of Thessaloniki, causing minor damage.

The building is near the site of the Thessaloniki Trade Fair, which Prime Minister Costas Karamanlis was to inaugurate and where he was to give an annual speech on the economy Saturday.

Greece has been plagued by bombings and arson attacks since the police shooting of a teenager in December, which sparked the worst riots the country has seen in decades.

Officials said they believe the latest attacks could be the work of left-wing militants who have carried out other attacks targeting police, banks, government buildings and a private television station.

The violence has embarrassed Karamanlis' government, which has been criticized for its inability to protect citizens.

The government has vowed to crack down on the increasing violence and sought advice from British police. (dpa)