Scuffles broke out in central Athens Sunday as tens of thousands of people protested a planned name change for neighbouring Macedonia that parliament is due to ratify this week.
After some 30 masked youths tried to force the closure of the parliament building by throwing stones, riot police responded with tear gas volleys to break up the crowd there.
Hundreds of buses, especially from northern Greece, had brought protesters in for the rally, on Syntagma square near the parliament.
Police estimates put the number of demonstrators at 60,000 at 1200 GMT, while organisers said 100,000 people had turned out.
The crowd was monitored by almost 2,000 police, equipped with drones and helicopters, a police source said. Much of the city centre was closed to traffic on Sunday and some metro stations closed as a precaution.
Greek clerics dressed in black joined the rally to protest an accord to rename Macedonia the Republic of North Macedonia, an agreement that would end a 27-year dispute with Greece over the country's name.
A wide range of Greek political parties, from the far-right to the socialists oppose the change, but it could nonetheless be approved by the required 151 deputies in the 300-seat parliamentary chamber.
"There is only one Macedonia, the Greek Macedonia, 'that's it'," read a sign in Greek and English held by Christina Gerodimoun, in her 30s.
"This government is a government of traitors," she said in reference to a coalition led by Prime Minister Alexis Tsipras, who brokered the deal with Macedonian counterpart Zoran Zaev in June.
Macedonia's parliament approved a constitutional revision to change the country's name 10 days ago. But for the deal between the two leaders to go through, the name change also needs to be approved by the Greek parliament.
Macedonia is a former Yugoslav republic, but for most Greeks, Macedonia is the name of their history-rich northern province made famous by Alexander the Great's conquests.
Tsipras' ruling coalition fell apart over the deal a week ago, but he then narrowly won a vote of confidence, setting the stage for parliament to vote on its ratification.