Protesters clashed with police outside Greece's Parliament at a rally that drew tens of thousands of people against the Greek-Macedonia name deal.
At least 25 police officers were injured and seven people arrested.
Demonstrators threw rocks, flares, firebombs, paint and other objects at riot police who responded with repeated volleys of tear gas. Some protesters jumped over a fence and tried to scale the steps, but officers chased them back down. One man draped in a Greek flag attacked police with a large stick, while others swung big flags on wooden poles and struck officers.
Greece's Parliament will debate this week on ratifying the deal and vote on it by Saturday. Macedonia's Parliament has already approved it, agreeing that the country would go by the name North Macedonia.
Macedonia and Greece struck the deal in June to end a decades-long dispute over Macedonia's name, which Greece says harbours territorial claims on its northern province of the same name.
Protesters are against the deal because they believe that any use of the name Macedonia in the neighbouring country's name is a usurpation of ancient Greek heritage and implies territorial claims on Greece.
Prime Minister Alexis Tsipras' office blamed "extremist elements and members of [far-right party] Golden Dawn" for the clashes.
A resolution of the name dispute would mark perhaps the most significant diplomatic step in the Balkans since the end of the Bosnian war. It would amount to a triumph for supporters of Europe, prevailing in the face of potent nationalist opposition while deepening integration.
Polls suggest that nearly 70 per cent of Greeks are against the accord, and many accuse Tsipras of overlooking national interests while pursuing a goal endorsed by US and Western European policymakers.
The name deal "is kind of a reminder of the liberal world order," said Angelos Chryssogelos, a research fellow at Harvard University. "But moving forward, you can wonder if the price to be paid is deepening scepticism toward elites."
The debate has become a touchstone of national identity in both Macedonia and Greece. .
"We cannot simply hand over our name, our identity, to the Slavs," said Georgia Tzeni, 32, from northern Greece who said the Greek Government is "giving away what generations have fought for. They are not listening to the people".
But the deal has widespread support from the West, and for months, US and European officials have shuttled into Athens and Skopje to push the accord along. Policymakers see the Balkans as a battleground in the struggle with Russia for influence and argue that Macedonia's EU membership would stabilise the region.
• Historic Macedonia encompassed a broad region that is now part of several nations.
• Greeks deny that the majority-Slavic population of their northern neighbour has a rightful claim to Macedonian heritage.
• The debate has raged for decades over everything from museums to airports named for Alexander the Great, king of ancient Macedon.
• The Republic of Macedonia is one of the states that formed from Yugoslavia and gained independence in 1991.
- AP, Washington Post