Several people collapsed from the effects of tear gas
Turkish riot police have used tear gas and water cannon to prevent protesters from staging a banned May Day rally in the centre of Istanbul.
The authorities say 505 people were arrested and several were hurt as crowds tried to reach Taksim Square.
Stones and bottles were thrown at security forces and police wearing gas masks broke up the demonstrators.
The Turkish government banned May Day celebrations in and around the square after 34 people died in 1977.
Turkey's three main union confederations had announced they would try to hold a rally, but eventually they gave up because of the clashes.
Police set up barricades around the square to enforce the ban, imposed after officials said they had received reports that radicals were planning violent protests.
Local media said six police and an unknown number of demonstrators were injured.
In Greece, workers marked May Day with a strike that severely affected train and ferry services. No newspapers were published and broadcasters operated with reduced staff.
Marches took place in Athens and other major cities as unions used the public holiday to protest against the Greek government's campaign of privatisation and pension reform.
In Belarus, a human rights group said 20 activists were arrested during a May Day protest in the capital, Minsk.