The largest gatherings occurred in Toronto, Montreal and Ottawa, where demonstrators espoused a myriad of causes.
They denounced exploitation of the working class and migrant workers, First Nations and nuclear energy. They took shots at Prime Minister Stephen Harper.
Union members, feminists and environmentalists joined with members of the Occupy movement, whose familiar "we are the 99 per cent" slogan could be seen on signs.
In Toronto, hundreds gathered outside City Hall, before marching down rush-hour Queen Street to a park under the watchful eye of scores of police officers.
"Workers around the world are what creates the wealth in society," said Darren Edgar as he marched.
"They should have the right to control and make the decisions to determine how our society is run."
The protesters also planned a 24-hour occupation but said they would clean up and leave by Wednesday evening.
Police said there were no incidents during the protest and march.
In Vancouver about 200 protesters rallied peacefully at the city's art gallery before marching through the downtown core.
A group of sixty gathered later at night and started a bonfire at an intersection. Officers moved in after protesters didn't listen to requests to disperse and the group quickly broke up.
Police said there were no arrests or injuries reported and the bonfire was put out.
The situation was different in Montreal, which has been the site of ongoing, and at times violent, student protests against tuition hikes.
At one point, some among the 1,000 protesters smashed windows, tossed objects at police officer who responded with chemical irritants, rubber bullets and billy clubs. Scores were arrested and one protester was injured.
Earlier in the day, one group of students was barred from crossing from Quebec into Ontario to join the May Day protest in Ottawa, union organizers said.
Some in the crowd of hundreds in Ottawa said they were supporting the Quebec students.
The main demonstration comprised a workers' march on Harper's downtown office. They then used hip hop, slam poetry and a little Aretha Franklin to protest budget cuts that are costing thousands of public-sector jobs.
As a puppet version of the prime minister bopped along to the songs, union vice-president Larry Rousseau led the crowd in a riff on Franklin's signature song "Respect."
"Their political promises may be sweeter than honey," Rosseau sang. "But guess what? So is my money. I'm telling you now, you don't mess with me. I want respect."
Several people sported buttons reading: "Stephen Harper hates me."
More than 12,000 workers have been notified their jobs are on the line as the government seeks to cut $5.2 billion in spending. A total of 19,200 positions are being cut.
Union president John Gordon said Tuesday's protest wouldn't be the last the Conservative government hears from the civil service.
"We're going to be in your face every single day because communities are losing services," Gordon said.
Around the world, May Day protesters vented fury over spending cuts, tax hikes and soaring unemployment.
In Asia, unions demanded wage increases, while in Europe, thousands of people filled the streets of cities in countries such as Greece, France and Spain to protest austerity measures.
Spain, with 24 per cent unemployment, is the latest focus of the eurozone debt nightmare that has already forced three countries to seek financial bailouts.
Even in Germany, where the economy is ticking along, unions estimated that 400,000 people showed up at May Day rallies.
There were also numerous protests in the U.S.
In Oakland, Calif., stinging gas sent protesters fleeing a downtown intersection where they were demonstrating. Officers took four people into custody.
Black-clad protesters in Seattle used sticks to smash downtown windows and ran through the streets disrupting traffic.
In New York, police in riot gear faced off against dozens of Occupy activists while about 50 demonstrators rallied in Chicago at a Bank of America branch.
— with files from Stephanie Levitz in Ottawa; Peter Rakobowchuk in Montreal; and the Associated Press
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