Police said about 15,000 people joined the Athens marches to vent anger at deep budget cuts which will hit public sector workers particularly hard. "It's the biggest attack on workers for centuries. They want to return us to the nineteenth century," said one protestor, printer Ericos Finalis.
In the northern city of Thessaloniki, police also fired tear gas at youths who attacked banks and businesses using iron bars during a protest that police said drew about 5,000 people. There were also clashes at a May Day march in the Chinese gambling mecca of Macau, where police used water cannon and pepper spray against hundreds of protestors who fought back with stones and bottles.
About 1,000 people joined the demonstration to demand job protection and action against illegal workers, Hong Kong's RTHK radio said. Tens of thousands filled a central square in Turkey's biggest city Istanbul for the first May Day celebrations at the site after dozens were killed there 33 years ago.
Waving colourful flags, the crowds marched into Taksim square, the hub of the sprawling city, and chanted: "Shoulder to shoulder against fascists." The square had been declared off-limits since the bloodshed during a May Day rally there in 1977 when gunmen, believed to be far-right militants aided by members of the intelligence services, fired on a crowd, triggering mass panic.
In Russia, diehard Communists hoisted red flags and portraits of wartime leader Joseph Stalin in a throw-back to Soviet-era parades and joining unionists to bemoan the economic crisis and call for a return to communism. An event in the country's restive North Caucasus region of Kabardino Balkaria was marred by a bomb blast that killed a 94-year-old veteran of World War II and injured nearly two dozen other people, according to news agencies.
May Day rallies also drew hundreds of thousands of people in several French cities although the turnout disappointed unions who had called for a massive show of force against pension reform. In Ukraine, thousands chanted slogans such as "Socialism is our future", and in Serbia activists warned about the "worsening situation for workers" while in Romania opposition supporters in red shirts and caps targeted the government.
German police rounded up about 50 leftists overnight for various incidents in the lead up to May Day. In Cuba, President Raul Castro presided over a May Day march staged as a show of unity in the face of what his government charges is a US and European-backed campaign to destroy the Cuban revolution.
The United States was meanwhile braced for demonstrations expected in more than 70 cities to protest at a tough new immigration law in Arizona that has triggered an outcry from rights groups. In Hong Kong, several thousand protestors marched through the streets to demand a minimum wage of 33 dollars (4.20 US) per hour and better job protection.
Thousands marched through the Indonesian capital Jakarta demanding better social security and shouting "Today we unite" and "Stop oppression now." Some 15,000 security personnel stood guard, using water canon to suppress a scuffle near the palace. The Indonesian social security system only covered about 25 percent of workers, Indonesian Workers Association head Saepul Tavip said.