Clashes have broken out in a number of countries as unions used traditional May Day marches to protest against the handling of the global economic crisis.
In the Turkish city of Istanbul, riot police fired tear gas and water cannon as protesters threw petrol bombs and attacked banks and shops.
Police made dozens of arrests in the German capital, Berlin, where protesters set fire to cars.
Greek police battled to quell rioters in Athens after banks were attacked.
There have also been marches in France and Russia, as well as the Philippines, Japan and Hong Kong, where demonstrators called on the authorities to do more to protect jobs.
Major demonstrations were also planned in Spain, Italy and Cuba.
"Workers are reiterating their demand that the government should find a way to stop mass lay-offs of workers and it should provide decent jobs, not short-lived jobs, not jobs for three months," said Leody de Guzman, of the Union of Filipino Workers.
Bottles and stones
Some 300 rallies are taking place across France, which has already seen strikes by hospital staff, fishermen and university staff, among others.
Across the country, tens of thousands of protesters took to the streets in cities such as Marseille, Bordeaux and Grenoble, ahead of a major demonstration in Paris.
This year's traditional Labour Day in France comes against a backdrop of mounting social tension, reports the BBC's Paris correspondent Emma Jane Kirby.
There is a growing perception that little has been done to protect the ordinary person's job and wages, while executives from banks bailed out by the government have enjoyed generous pay-offs and bonuses, she says.
The country's eight main unions have urged people to come out and protest in their third such day of action this year.
Violence erupted in Istanbul as hundreds of left-wing and trade union groups tried to pass through police checkpoints into the city's main Taksim square.
The protesters had been refused permission to hold rallies in the square but, as in previous years, they chose to ignore the ban, reports the BBC's David O'Byrne in Istanbul.
The marchers took to the back streets after they were met with police water cannon, and hurled stones and other missiles at police who responded by firing tear gas.
German police look on at burning dustbins after clashes in Berlin on 1/5/09
Rubbish bins were set alight during clashes in Berlin
A large number of people are believed to have been arrested and there is considerable damage to shops and vehicles, but no serious injuries, our correspondent reports.
In Berlin, demonstrators clashed with police. Bottles and stones were thrown at police, passing cars and trams, and rubbish bins were set alight.
The unrest began when some 200 protesters began chanting anti-capitalism slogans after a street party ended in the early hours.
Twenty-nine police were injured, and at least 12 people were arrested.
Violence has been a feature of past May Days in Germany. Some 5,000 police are set to be deployed in Berlin.