Athens argues that the name Macedonia suggests that Skopje has territorial claims to the northern Greek region of the same name, of which Thessaloniki is the capital.
Hardline clerics, far-right leaders and Greek diaspora groups have called for rallies, and media reports say 30,000 people are expected to demonstrate in Thessaloniki alone, with more than 400 buses bringing people in from across the country.
Cretans in traditional costumes who have travelled from the southern island with their horses, as well as people from northern Greece wearing costumes from the Macedonian wars era a century ago, were already gathered at the White Tower on the Thessaloniki waterfront ahead of the protest's planned start at 1200 GMT.
Greece and Macedonia returned to the United Nations last week hoping to reach a compromise that could end the 27-year dispute over the former Yugoslav republic's name.
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Greece's objections to the use of the name Macedonia since the Balkan country's independence in 1991 have hampered the tiny nation's bid to join the European Union and NATO.
The UN negotiator Matthew Nimetz -- a 24-year veteran on the issue -- said last week that he was "very hopeful" that a solution was within reach.
The Greek region of Macedonia boasts the major port cities of Thessaloniki and Kavala and was the centre of Alexander the Great's ancient kingdom, a source of Greek pride.
In 1992, more than one million people -- 10 percent of the population -- joined a rally in Thessaloniki to proclaim that "Macedonia is Greek".
According to a survey conducted for Greek radio station 24/7 by the Alco polling group, 63 percent of respondents said they thought it was in Greece's best interests to seek a mutually acceptable solution at the UN talks.
And the Greek Orthodox Church, which is traditionally opposed to the use of the term Macedonia from Skopje and led the 1992 rally, appears to have distanced itself from Sunday's events.