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Riot police in Turkey have used tear gas and water cannon to prevent demonstrators defying a ban on protests on Istanbul's central Taksim Square.
The Anadolu news agency said several demonstrators were injured and at least five detained.
Intensive security measures were in place, and roads and streets near Taksim Square closed to traffic from the early morning.
Turkish media said some 40,000 police would be deployed in the city on 1 May.
Prime Minister Recep Tayyip Erdogan earlier warned people they should "give up hopes" of gathering on Taksim Square - a focus of anti-government protests.
But a joint statement from the main trades unions on Wednesday said: "We will be in Taksim despite the irrational and illegal ban. All roads will lead to Taksim on May Day".
Mr Erdogan's party won local elections in March. That was the first vote since mass protests last June, and was seen as a barometer of his popularity.
The prime minister has been eyeing a run for the presidency in August - the first time voters will directly elect the head of state - or may seek to change the rules to allow him to seek a fourth term in office.
Elsewhere in May Day celebrations:
A number of people are reported to have been beaten by security forces in Cambodia who were trying to break up an opposition march in the capital Phnom Penh
More than 100,000 Russians have taken part in a parade in Moscow's Red Square, reviving a tradition last seen before the collapse of the Soviet Union in 1991.