Thousands of demonstrators are gathering for an annual protest march to the U.S. Embassy in Athens, to commemorate a student uprising that was crushed by the country's military regime in 1973.

Police said about 2,500 people were outside the Polytechnic building in central Athens, before the march that has been marred in previous years by violence from far-left protesters.

Earlier Tuesday, leftists heckled Prime Minister Alexis Tsipras when he laid a wreath at the site where most of the students killed by security forces in the uprising died.

Riot police are providing strong security for the march. Protesters will walk from the Polytechnic to the U.S. Embassy, as many Greeks blame Washington for backing the dictatorship that ruled Greece from 1967-74.