The Greek parliament is expected to vote January 25 on a historic agreement to normalize relations with neighboring Macedonia.
As the Greek parliament debated the deal on January 24, demonstrators gathered outside to protest, some of them chanting "traitors."
Police fired tear gas and stun grenades after some in the crowd hurled rocks and other objects in their direction.
Police later said they arrested 10 people and detained another 133 on suspicion of committing or planning acts of violence. A new protest has been called outside parliament for January 25.
Under the agreement, Macedonia will change its name to the Republic of North Macedonia. The deal could unblock Macedonia's bids to join NATO and the European Union, long blocked by Greece.
The accord has already been ratified by Macedonia's parliament, but still needs Greek parliamentary endorsement.
The agreement is backed by Western countries that want to limit Russian influence in the Balkans. But it has faced opposition, mainly from nationalists in both Greece and Macedonia who argue it gives away too much to the other side.
Inside parliament on January 24, Prime Minister Alexis Tsipras urged lawmakers to approve the deal.
"We are one step before a historic event," he told them.
"Nationalism in the Balkans has led to disasters," Tsipras said. "And I believe the time has come to escape nationalism."
Tsipras's government is expected to get support from a small number of opposition lawmakers to secure the 151 votes needed in the 300-member parliament for the agreement to be ratified.
Opposition leader Kyriakos Mitsotakis rejected the deal as a "national defeat." The leader of the New Democracy party threatened to block Macedonia's EU membership drive if his party triumphs with voters in elections later this year.
The ratification vote in parliament originally was scheduled for January 24. Debate was extended until January 25 to allow the large number of registered speakers to have their say.