Thousands of NATO protesters took to the streets of Chicago Friday chanting "tax Wall Street!" and calling for an end to costly wars so governments can focus on providing social services.

The protesters blocked traffic in the heart of Chicago's financial district as the city's highways were being shut down to make way for the first motorcades bearing the leaders of 50 countries gathering for a NATO summit on Sunday and Monday.

"Whose streets? Our streets!" hundreds of protesters chanted as they marched past police mounted on horses and bicycles, while onlookers took pictures from the sidewalks and through the windows of shops and offices.

Fears that the protests could turn violent have put the city on edge, with some downtown businesses even telling office workers to ditch their suits and ties and dress down to avoid being hassled or targeted on the streets.

Police and protest organizers have vowed that there will be no repeat of the trouble that erupted at G20 summits in London and Toronto or the riots that scarred Chicago during the 1968 Democratic National Convention.

The decision to move the G8 summit -- set for Friday and Saturday -- from Chicago to the presidential retreat of Camp David outside Washington is expected to lessen the intensity of demonstrations in President Barack Obama's adopted hometown.

The impromptu -- and unsanctioned -- march was a mostly cheerful and disorganized affair, though some bandana-clad marchers chanted obscenities about the police and used sticks to drum on lamp posts.

It began after protesters left a formal rally where hundreds of nurses wearing Robin Hood hats and red t-shirts demanding "an economy for the 99 percent" tried to call attention to a global movement to enact a tax on financial transactions such as stock, bond and derivatives sales.

"NATO is here, the time is right for dancing in the streets," they sang as the crowd broke out into a dance routine.

"They'll be teachers teaching and nurses healing -- dancing in the streets... it's time to tax Wall Street!"

Sharon Tobin, a nurse from San Francisco, laughed and shouted "let's do that again" when the song came to an end.

"We were practicing and practicing and practicing," she told AFP. "Demonstration or no demonstration, we always have fun."

Over 100 other groups -- including delegations from Europe, Canada, South Korea and Guatemala -- joined the rally, arguing that a Robin Hood tax would raise an estimated $350 billion a year to pay for health care, education and other basic needs and services.

Craig Nisbet, 27, came down with a group of friend from Toronto to spend the weekend protesting and ended up sleeping on the street Thursday night.

He laughed and hugged people as he wandered through the crowd holding a signs saying "NATO NO WAY TO."

"The money that's spent on militarization and war these days compared to social services, health care and education is so wrong," Nisbet said.

"NATO are basically the military enforcers of G8 policy and the G8 is one of the most undemocratic systems there is."

Chris Phillips said he was heartened to see so many different groups standing together "to end imperialism, to end globalization and stop the destruction of third world nations for corporate profit."

"That has to end immediately and that's why we're here," Phillips said.

"Every time Congress can't get approval to launch another war on another third world nation, they turn to NATO and NATO does the job."

Adrianne Hernan said it's time to complete disband the transatlantic military alliance, saying it has outlived its initial purpose of collective defense and has become a force of economic imperialism.

"After the disbanding of the Soviet Union ,there really isn't much use for it, but it runs around kind of like its own super power and goes around takes resources," she said.

"It's a tool of the one percent to gain more resources and to enrich themselves."

An estimated 3,000 people joined the rally and marches Friday.

A much larger number is expected Sunday when protesters will march to the lakefront convention center where the summit is being held.