Tens of thousands of Greeks are protesting against a possible compromise in a dispute with Macedonia over its name.
Around one million people are expected in Athens’ Syntagma Square, many travelling from outside the city or even overseas.
The protest is part of a 27-year disagreement between the Greek province of Macedonia, of which Thessaloniki is the capital, and the country of Macedonia.
In 1993, Macedonia joined the United Nations as the Former Yugoslav Republic of Macedonia – or FYROM – after declaring independence from the former Yugoslavia two years earlier.
Greece has opposed the name, saying its region has been named Macedonia since Alexander the Great – also known as Alexander III of Macedon – ruled from 336 BC.
Athens worries that the name could mean Skopje can make claims to Greek territory and cultural heritage.
UN mediator Matthew Nimetz has been meeting both sides and has suggested alternative names, including “Republika Nova Makedonija” (Republic of New Macedonia).
Greek Prime Minister Alexis Tsipras is considering this.
Mr Tsipras’ government has said it is ready to accept a name that will establish a clear distinction from Greek Macedonia.
But many Greeks are unhappy about allowing Macedonia to keep anything close to its current name.
The dispute is preventing Macedonia from joining bodies such as NATO, as Greece refuses to back its bid until the name issue is solved.
James Ker-Lindsay, an expert in southern European politics at St Mary’s University, told Sky News: “There’s a real sense that, with a new government that has come to power in Macedonia last year, this is an opportunity to turn a page, to turn a corner for the country and to start it on a process of European Atlantic integration.”
But he said that the dispute goes beyond just the name of the country, adding: “It’s also about the adjective…It moves beyond the name to much deeper issues of identity in the region.”
Allia Sarellis, who flew from the US to join the protest, said: “Macedonia is Greek and only Greek.
“They are trying to steal history.”
Meanwhile, tens of thousands of police officers have been deployed to supervise the protest, and an anarchist-planned counter-protest nearby.
Mr Ker-Lindsay said: “The real worry (at the protest) is that fringe elements would use this opportunity to start attacking police, use firebombs.
“You can never really discount that, as there is a strong tradition of it in Greek politics.”
Last month, at least 90,000 people joined a similar protest in Thessaloniki.