The history of the commemoration goes back to the 1886 Haymarket incident in Chicago, when a number of protesters - trade unionists and common workers of varying political beliefs - were massacred by police, or hanged after show trials.
Later, in 1890-1 the Socialist International recognized May 1 as an international day to celebrate the struggle for workers' rights.
Massively celebrated in the former Eastern Block, May 1 is also immensely popular with people across Western Europe, with workers traditionally gathering in large rallies on that day.
In Britain, May Day public holiday falls on the first Monday in May !
May 1 is a national holiday in more than 80 countries. It is also celebrated unofficially in many other countries.
Right-wing governments have traditionally sought to repress the message behind International Workers' Day, with fascist governments in Portugal, Italy, Germany and Spain abolishing the workers' holiday, and the Conservative party in the UK currently attempting to abolish the UK's annual May Day Bank Holiday.
Message to Cameron: (British Prime Minister)
Labour movement activists warned killjoy Prime Minister David Cameron today to "keep your hands off our May Day holiday."
They urged protests against the Tories' vindictive scheme to spoil the workers' extended weekend of celebrations by abolishing the May Day bank holiday.
Mr Cameron and his Cabinet of millionaires are hoping to make this Monday's May Day bank holiday one of the last.
Plans are afoot to abandon the holiday on the first Monday in May, and replace it with a holiday on St George's Day April 23, or the anniversary of the bloody Battle of Trafalgar on October 21.
Veteran Labour MP Dennis Skinner urged the labour movement to band together "to ensure that we keep the May Day flags flying as they do across the world."
Mr Skinner warned: "In sharp contrast to the pomp and circumstance and organised happiness around the royal wedding, the Tory establishment is now turning its attention to ruining the workers' traditional May Day celebrations.
"True to type, Cameron wants to create more unhappiness for working class people who are struggling to keep a job.
"He wants to stop them from enjoying the May Day bank holiday and from celebrating International Workers' Day along with people across Europe and around the world."
Rail union RMT general secretary Bob Crow said: "The most important events this weekend are the annual May Day celebrations - not the bread and circuses that were rammed down people's throats."
Declared Mr Crow: "Forget all the diversions. The trade union and socialist focus is on marking International Workers' Day at a time when the working class is under unprecedented attack."
He warned that the Tories were not only attacking the workers, but were also gearing up for an assault on May Day itself.
"Send out the message to the Con-Dems and the bosses loud and clear - la lucha continua!" urged Mr Crow.
Left Labour MP Kelvin Hopkins, who spent today tramping the streets of Luton delivering election leaflets, accused the government of blatant right-wing tactics in seeking to ditch the May Day holiday.
Unite union general secretary Len McCluskey said: "The Tory attack on the May Day holiday is to be abhorred and it's another reminder that this government has scant regard for the working people of this country."
May Day was an accepted holiday in many countries to commemorate workers' struggles, said Mr McCluskey, who suggested an extra bank holiday in October during the "long stretch" between August and Christmas.
General union GMB general secretary Paul Kenny had a blunt message for the government of millionaires: "Keep your hands off our May Day. Keep your hands off workers' day."
Mr Kenny added: "May Day is an important celebration of the achievements of humanity. We have got to fight to save the bank holiday."
Communist Party of Britain general secretary Rob Griffiths said International Workers' Day celebrated "the commitment to solidarity, peace and socialism."
But a change to Trafalgar Day would be celebrating "the slaughter of seafarers in yet another battle in Britain's long and bloody history of imperialist wars."