Aaron Smith and Julianne Pepitone
NEW YORK (CNNMoney) - Occupy May Day is underway.
Occupy San Francisco got an early start on May Day, kicking off its organized protest at 8 p.m. local time on April 30.
"San Francisco, once a stronghold of the dispossessed, has become a playground for the rich and a living hell for those of us who can't keep up or have no interest in capitalist relations," Bay Area protest organizers said in a statement.
But morning rush hour traffic was still flowing on the Golden Gate Bridge, despite protesters' threats to shut it down.
Early Tuesday morning, protesters -- including Occupiers, labor unions, and community and religious organizations -- were picketing against unfair labor practices at Los Angeles International Airport, according to Occupywallst.org.
The L.A. airport issued a traffic advisory, and the airport's website said two blocks of nearby Century Boulevard were shut down in anticipation of a march by at least 1,200 union members at 3 p.m. ET.
May Day protests were also underway in Greece and Turkey. Two thousand people gathered in Syntagma Square in Athens, and another 7,000 protesters gathered outside a factory where employees haven't been paid in six months, according to their union.
Thousands of protesters in Istanbul's central Taksim Square were met with a police presence.
A march was planned in London, ending at Trafalgar Square, and road closures were also in the works. Three men were arrested in Exchange Square outside Liverpool Street.
In New York, Occupy Wall Street has been planning protests at bridges and tunnels, with picket lines slated to kick off at 8 a.m. at the Chase Building, the New York Times building, Sotheby's and Manhattan's main U.S. Post Office.
Protesters were gathering at Bryant Park in midtown Manhattan for a planned march south to Union Square. The turnout was relatively slim, with about 100 protesters chanting, beating drums and making signs and T-shirts. In addition, Occupy Wall Street said that approximately 150 protesters were marching from Brooklyn to Manhattan via the Williamsburg Bridge.
"The popular media narrative is that Occupy is dead, but what we're here to show is that that's far from the truth," said Occupy Wall Street spokesman Mark Bray. "The issues that we're talking about are too important to go away."
Bray said that Occupy has united with workers' unions and immigrant rights groups "to show that the 99% is what really drives this country."
May Day has historically been a day of protests, with workers currently marching in Moscow and Havana, and protests underway in Madrid.
This year, the Occupy movement has organized protests in 125 U.S. cities, according to Occupywallst.org. The theme is a general work strike to create "a day without the 99%:" no work, no shopping, no banking.
Protesters in Oakland, Calif., had originally planned an attempt to shut down the Golden Gate Bridge in support of a local labor union. That plan was called off on Sunday, but organizers in the San Francisco Bay Area have scheduled picket lines and many other demonstrations.
Occupy organizers have called specific attention to Chicago because the city is hosting the NATO Summit on May 20. The magazine Adbusters, which spearheaded the original Occupy Wall Street event, called Chicago "the focal point of this global spiritual insurrection" because of the summit.
History of Occupy: Occupy bills itself as leaderless, and opponents immediately criticized it for a perceived lack of concrete goals. But it found solidarity in labor unions and other activist groups, and the movement spread quickly through social media.
The first Occupy Wall Street protest began when hundreds of protesters descended on the nation's financial center last September.
A few weeks after it began, Occupy Wall Street faced the first of a series of evictions from its home base in a lower Manhattan park. But protesters regrouped in other areas.
And by then, solidarity protests had popped up in San Francisco, Denver, Los Angeles, Boston and Chicago, among other cities.
The relative calm of the protests began to shatter in October, when hundreds were arrested on the Brooklyn Bridge as part of Occupy Wall Street.
Also in October, the site in Oakland became violent when police used tear gas after protesters reportedly threw paint and other objects at officers. Major clashes between police and protesters have taken place in Oakland since that event, and hundreds of people have been arrested.
Critics questioned whether those skirmishes would derail Occupy, and whether protesters would be willing to remain outside in the winter. But Occupy sites in many cities remain active -- perhaps helped by unseasonably warm weather.
--CNN's Poppy Harlow contributed CNN International's Maggie Lake and Ivan Watson contributed to this report.