Swiss police fired rubber pellets and German riot police used tear gas and water cannon as rock-throwing anarchists and other violent protesters marred May Day rallies in some European cities.
Rallies around the world were largely peaceful however, with hundreds of thousands of workers marking the day and protesting over labour and political issues. Some leftists used it to condemn the US-led war on Saddam Hussein's Iraq.
In Beijing, the SARS crisis dampened the normal huge May Day celebrations as millions of workers stayed at home to avoid catching the flu-like disease that has killed 159 in China.
About 150 members of the once-banned Iraqi Communist Party gathered outside a Baghdad hotel to mark the occasion. Under Saddam, any May Day gathering was state-controlled.
Police were out in force in Berlin, deploying about 7,500 officers on the streets after 27 police and an unknown number of attackers and bystanders were injured in three hours of clashes overnight.
The violence erupted at the end of a peaceful protest by about 4,000 people in a park where the Berlin Wall once stood.
Some 200 anarchists hurled bottles and rocks and set off fireworks at police, who charged the group and arrested 97. Street barricades were set ablaze and shop windows smashed.
In the Swiss financial centre Zurich, around 100 masked leftist protesters threw stones and bottles at police, who responded with rubber pellets. Shop windows were wrecked and cars damaged.
Some 7,000 people staged a peaceful march in the city earlier to condemn the war in Iraq and criticise so-called "fat cat" managers drawing huge pay packets while the global economy struggles.
"Yesterday Afghanistan, Today Iraq, Tomorrow...?" read one banner.
Police in Berne used water cannon to stop some 300 demonstrators from entering a government-controlled weapons firm. The protesters demanded a ban on arms exports.
In Istanbul, about 300 demonstrators clashed with Turkish police as they tried to hold an unauthorised demonstration in a central square. Around 30 were arrested.
In another part of Istanbul, 25,000 demonstrators, trade unionists and members of Kurdish groups marched peacefully, calling on the government to withdraw employment laws that unions say reduce workers' rights.
In London, May Day marches by anti-capitalist demonstrators went ahead largely without incident despite anarchists preparing a hit-list of more than 50 "companies of mass destruction".
Brief scuffles and a sit-down protest by 150 demonstrators took place outside an office of U.S. arms manufacturer Lockheed Martin after a police officer was struck by a beer can.
The anarchists' hit-list, posted on the Internet, included the names and addresses of oil companies, arms manufacturers, banks, multinationals and government departments such as the Defence Ministry.
Police mobilised 4,000 officers in the capital to guard against violence and all leave was cancelled.
In Madrid, a rally calling for world peace and workers' rights was marred when the head of Spain's biggest trade union, Jose Maria Fidalgo, was beaten on the head by a protester angry at the union. Fidalgo was taken to hospital for stitches.
In Russia, pro-Kremlin parties and trade unions stole the show from the fading communist opposition by organising a rally in central Moscow that drew an estimated 25,000 people.
The peaceful, flower-waving crowd marched down Moscow's main thoroughfare, joined by the city's mayor Yuri Luzhkov and former Soviet President Mikhail Gorbachev.
Tens of thousands of workers turned out in Italy and Bulgaria, and hundreds of thousands of unionists took part in rallies in Japan.
In Seoul, about 20,000 people joined a rally demanding a shorter working week, better conditions and job security.
Around 10,000 marched in Manila, some wearing masks to symbolise what they saw as the harmful effects of globalisation on local industries.