In tense Istanbul, police dispersed hundreds of protesters who tried to defy a ban on demonstrations on the city's Taksim Square on the anniversary of clashes that spawned a nationwide protest movement.
The square has been the scene of protests that have dogged the government for months.
After giving a final warning, hundreds of riot police backed up by water cannon moved in on protesters in the Besiktas district as they tried to breach the barricades leading up to the symbolic square, according to an AFP reporter.
Russian workers, meanwhile, were to parade on Red Square for the first time since 1991 - the latest Soviet tradition to be revived as a wave of patriotism sweeps the country.
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May Day was a key date in the Soviet calendar, with elaborate celebrations involving ranks of marching athletes, soldiers and workers on the Moscow square, but in recent years the annual demonstrations have been relegated to a city highway.
The park, opened by the government in 2010 as a designated area for people to air their grievances, was closed off by police with barbed wire as the authorities seek to clamp down on protests against long-ruling strongman Prime Minister Hun Sen.
"We are sad that we could not mark May Day properly. Workers' rights have been thwarted," said Ath Thorn, president of the Coalition of Cambodian Apparel Workers Democratic Union.
"Demonstrations will be held nationwide but the biggest will be in Jakarta, with around 33,000 workers," police spokesman Rikwanto told AFP, adding that 18,000 police officers would be out on the streets.
He said workers were planning further rallies on Friday, with some 10,000 protesters expected to turn out.