Greek communists rally in Thessaloniki. May Day comes against a backdrop of key elections
Labour demonstrations marking May Day are taking place across the world, with the main focus on Europe and its backdrop of unpopular austerity measures and rising social unrest.
Greece, Spain and Portugal are set to hold large nationwide demonstrations.
At a Paris rally, National Front leader Marine Le Pen is expected to tell her supporters who they should vote for in Sunday's presidential run-off vote.
The Occupy protest movement has urged May Day action spanning the globe.
Anti-austerity protesters will take part in a day of strikes and demonstrations across Greece.
The BBC's Mark Lowen in Athens says the 1 May protests there have become an institution, with public and private sector strikes.
In the capital, Athens, buses and trains came to a halt in a 24-hour strike, and there is a four-hour stoppage on the ferries.
But our correspondent says there may be less of the traditional violence, since minds are focused on Sunday's general election, when many Greeks are expected to vent their anger against the government's austerity measures.
Elections on Sunday are also the main focus in France.
Ms Le Pen will lead a march in Paris to the statue of Joan of Arc, an iconic figure for the far right.
She has promised to indicate where the 6.5m voters who supported her in the first round of the presidential election should cast their votes on Sunday, in the crucial second round between President Nicolas Sarkozy and the Socialist candidate Francois Hollande.
The BBC's Gavin Hewitt says the expectation is that she will advise them to abstain.
Mr Sarkozy will rally in Trocadero Square, while French unions march to the Bastille.
Mr Sarkozy has dubbed his rally a showcase of "real work".
Mr Hollande said Mr Sarkozy was more the president of "real unemployment".
The main May Day rally in Spain is expected to get under way in Madrid at about 10:00 GMT, while Portugal's labour unions will rally in the afternoon.
In Russia, nationalists, communists and opponents of incoming president Vladimir Putin are all holding separate rallies.
Mr Putin and outgoing President Dmitry Medvedev made a rare joint public appearance on the streets of Moscow, leading more than 100,000 people in a Soviet-style "Holiday of Labour and Spring" march.
The Occupy movement has called for global protests against economic inequality.
The movement gained international attention with the Occupy Wall Street protest last September but has struggled to maintain its profile as its supporters began to be evicted from public squares across the US.
An Occupy statement said: "The Occupy Movement has called for A Day Without the 99% on May 1st, 2012," referring to its slogan that the wealthy 1% rules over a powerless 99%.
Its main rally will be in New York in the afternoon rush hour.
The Occupy movement in San Francisco called for a Golden Gate Bridge protest.
It said: "This May Day we look forward to seeing strong, powerful picket lines, unlike anything the Golden Gate Bridge bosses have seen before."
Rallies have already taken place across Asia, including:
In Hong Kong, about 5,000 workers marched demanding a rise in the minimum wage
In Jakarta, Indonesia, more than 9,000 workers marched to the state palace calling for better pay and job protection
In Manila, the Philippines, some 8,000 workers rallied near the Malacanang palace to call for pay increases.