Protesters say Athens is compromising over Macedonia's name, arguing it suggests that Skopje has claims to the territory and heritage of Greece's northern region of the same name.
Tens of thousands of Greeks were expected to urge the government on Sunday not to compromise in a festering name row with neighbouring Macedonia, at a mass rally in Athens that organisers vow will be "majestic".
Athens objects to Macedonia's name, arguing it suggests that Skopje has claims to the territory and heritage of Greece's historic northern region of the same name.
However, leftist Greek Prime Minister Alexis Tsipras has been considering a resolution to the 27-year-old dispute, angering many opposition members and his own nationalist coalition partners.
Hours before Sunday's rally was scheduled to start at 1200 GMT, crowds of flag-bearing Greeks were heading towards central Syntagma square in the capital.
Organisers said they expected around a million participants on Syntagma Square, with many travelling from across the country or even abroad.
"Macedonia is Greek and only Greek. They are trying to steal history. We all have to fight and let the world know," said Allia Sarellis, a member of the Greek diaspora who flew in from the United States.
Around 2,500 buses from northern Greece and two ships from Crete were bringing thousands of demonstrators, reports said.
Police declined to give numbers.
Several protesters wore traditional garb, including the uniform of Greek guerrillas who fought Bulgarian bands and Ottoman forces in Macedonia in the early 20th century.
"Thousands of Cretans have shed their blood for Macedonia," a bearded Cretan protester told Skai TV.
The protest - the second on the Macedonia issue in a fortnight - has been organised and funded by Greek diaspora groups, with the support of retired officer associations, cultural unions and church groups.
"It will be majestic... It will send a very strong message both inside and outside Greece," rally spokesman Michalis Patsikas told reporters midweek.
Anarchists were planning a counter-protest nearby, with riot police deployed to keep the two crowds apart.
Two weeks ago, tens of thousands had protested in Thessaloniki, the capital of Greece's Macedonia region. Police had estimated the turnout at over 90,000, while organisers claimed at least 400,000 attended.
"(Turnout) will be at least triple that of Thessaloniki. There are no tickets left for people to come from Thessaloniki and Crete," Patsikas said.
The dispute has remained unresolved since the former Yugoslav republic's independence in 1991.
Greece considers the name "Macedonia" to be part of its own cultural heritage, as the province was the core of Alexander's ancient empire.
Athens seeks guarantees that the use of the name by its neighbour implies no claim to parts of its own territory.
The government has accused far-right hardliners and the neo-Nazi Golden Dawn party of trying to exploit the issue.
Georgia Bitakou, a journalist and former fight promoter, insists the rally is non-political.
"We are all patriots. There are no parties, no colours. Our only goal is to give a mandate to (state) representatives," she said.
Athens says it is ready to accept a composite name that will establish a clear distinction from Greek Macedonia.
"The government is trying to give a patriotic solution to a problem that has troubled the country, its international relations and its diplomacy for over 25 years," government spokesman Dimitris Tzanakopoulos said on Sunday.
"Not having a solution undermines our national interest," Prime Minister Alexis Tsipras said last week.
Because of Greece's objections, Macedonia in 1993 joined the United Nations as the Former Yugoslav Republic of Macedonia.
To break the deadlock, longterm UN mediator Matthew Nimetz has now proposed several alternative names in Macedonian, including "Republika Nova Makedonija" or the "Republic of New Macedonia".
A resolution of the issue is needed before Macedonia can join NATO or the EU.