A group of university students heading the annual march marking the anniversary of a bloody 1973 student uprising against the military dictatorship sang the national anthem outside the American Embassy in Athens on Saturday.
The students, carrying a blood-stained Greek flag, chanted slogans, then folded the flag and left.
The Communist Party and other groups who have gathered in several points in central Athens are holding separate marches towards the embassy.
Earlier on Saturday, students, teachers and pensioners laid wreaths and carnations at a monument in the Athens Polytechnic University to honor the dozens killed in the 1973 revolt.
More than 5,000 police officers have been assigned to patrol the city before and during the big march from the historic university building on Patission Street to the embassy across town at Mavili Square, while police is also using drones to monitor the march by air.