Athens police have fired teargas during a clash with anti-war demonstrators protesting against a visit by US Secretary of State Condoleezza Rice.
TV pictures showed the air thick with teargas
Some protesters hurled petrol bombs, sticks and stones in return.
A Greek Communist leader accused Ms Rice of drumming up support for an attack on Iran, which the US accuses of trying to build a nuclear bomb.
Ms Rice met Greek Prime Minister Costas Karamanlis before going on to Ankara to meet her Turkish counterpart.
While in Athens, Ms Rice said Iran was isolating itself from the international community by threatening to suspend co-operation with the UN nuclear watchdog, the International Atomic Energy Agency, if sanctions are imposed.
Protesters tried to reach the buildings where Ms Rice met Mr Karamanlis and her Greek counterpart, Foreign Minister Dora Bakoyianni, but many retreated under the police pressure.
Television pictures showed some protesters throwing petrol bombs and using sticks as riot police advanced, the air thick with tear gas.
Some 5,000 riot police were deployed for Ms Rice's visit
Self-styled anarchists trailing the demonstrators continued fighting police, burning cars and smashing shop fronts, Reuters news agency reported, but there were no reports of injuries or arrests.
Police helicopters circled the city centre.
"We are not protesting just against Rice, but the imperialist, war-mongering US government," school teacher Panayiotis Hiundis told Reuters.
At least six people were detained on Monday after they managed to unfurl a giant banner reading "Condoleezza Rice go home" on the Athens concert hall, adjacent to the US embassy, Associated Press news agency reported.
The US war in Iraq has triggered strong opposition in Greece. In addition, say commentators, many Greeks are still bitter at Washington for backing the right-wing military junta which ruled Greece between 1967 and 1974.
Ms Rice told reporters accompanying her to Europe that she would discuss Balkan peacekeeping, the future of the divided island of Cyprus, and the threat of terrorist attacks along the Turkey-Iraq border.
The date of Ms Rice's visit was changed to try to circumvent huge rallies against the wars in Iraq and Afghanistan which were planned to coincide with it.