Some 8,000 riot police were deployed in Athens
Greek riot police have fired tear gas to disperse protesters hurling stones and firebombs outside the US embassy in the capital Athens.
The clashes took place during a march by some 10,000 people to mark a 1973 student uprising that was crushed by the then ruling military government.
Dozens of students are believed to have been killed during that uprising.
Every year, demonstrators march to the US embassy to protest against American support for the junta at the time.
The clashes began after a group of about 100 people broke away from the peaceful march in Athens on Monday.
Greek police fired volleys of tear gas at the protesters who were throwing stones and Molotov cocktails at a police cordon protecting the US embassy.
Three policemen were injured and 11 protesters were arrested, Greek officials were quoted as saying by the the Associated Press.
Some 8,000 police were deployed to guard the protesters, many of whom chanted slogans against the US "war on terror", the banking industry and the conservative Greek government.
A big demonstration was also held in Greece's second largest city, Thessaloniki.
The 1973 student revolt was crushed when the Greek junta sent tanks into the campus of Athens Polytechnic University.
Dozens of students are said to have been killed in the clashes, although the exact death toll has never been established.
The student revolt heralded the end of the 1967-1974 junta in Greece.