ATHENS Nov 17 (Reuters) - Thousands of Greeks marched peacefully through the capital Athens on Wednesday to commemorate a 1973 protest in which dozens died when the army cracked down on students opposing military rule. Hundreds of police guarded the peaceful march to the Embassy of the United States, which provided tacit support to a seven-year military dictatorship that collapsed in 1974. At the front of the procession, individuals held a blood-stained Greek flag which belonged to students engaged in the revolt that triggered an army crackdown. Dozens are thought to have been killed when army tanks smashed through the gates of the Athens Polytechnic in 1973, where students had been staging a protest against the colonels ruling Greece since 1967. The junta unravelled in 1974, amid a public outcry over a coup they instigated in Cyprus, triggering Turkey"s invasion of the island just days later. The annual march is often a focal point for the public to vent anger at authorities. More than 20,000 people participated in the demonstration, which brought most of central Athens to a standstill. Police deployed about 6,000 officers and two helicopters hovered over the city centre.