It has been a long-held dream of New Yorkers of a certain (greenish) stripe: the streets of Manhattan free of cars. Now, for a few hours, on a few streets, on a few weekends this summer, that dream will become reality.

Mayor Michael R. Bloomberg announced on Monday that he will create a car-free zone on three Saturdays in August, along a 6.9-mile stretch of streets through Manhattan, from the Brooklyn Bridge, north to Park Avenue and the Upper East Side. Cars, trucks and buses will be banned on the streets along the route from 7 a.m. to 1 p.m. on Aug. 9, 16 and 23. The mayor was careful to describe the initiative, called Summer Streets, as an experiment.

“If it works, we’ll certainly consider doing it again,” Mr. Bloomberg said, at a news conference in the East Village on Lafayette Street, which will be included in the route. “If not, we won’t. But we have never been afraid to try new ideas, especially the ones that have the potential to improve the quality of life.”

The route will run north-south along Centre Street, Lafayette Street, Fourth Avenue and Park Avenue to 72nd Street. The southern half of 72nd Street from Park Avenue to Fifth Avenue will also be shut to vehicles, to link to Central Park.

Mr. Bloomberg and the transportation commissioner, Janette Sadik-Khan, said the idea was to make the streets a haven for walkers, cyclists and others. Fitness, dance and yoga classes will be held along the route, and there will also be places to rent bicycles. The plan was reported in The New York Sun on Monday.

The musician David Byrne, left, joined Transportation Commissioner Janette Sadik-Khan,

Mayor Michael R. Bloomberg,

the bicyclist Lance Armstrong

and the Manhattan borough president, Scott M. Stringer,

on Monday for the announcement of “Summer Streets.”

(Photo: John Marshall Mantel for The New York Times)

“It’s a new way to use a street, using it more as a park than as a thoroughfare,” said Paul Steely White, the executive director of Transportation Alternatives, an advocacy group promoting walking, bicycling and mass transit that worked with the city to develop the car-free zone.

“Everyone around the world knows about Park Avenue as one of New York City’s most storied thoroughfares, and to turn that over to pedestrians and cyclists, even though it’s just for three consecutive Saturdays, I think that sends a very powerful message that the tide is turning so that bicyclists and pedestrians are on at least an equal footing with drivers.”

While the idea seems novel in New York, it has been tried with success in many other cities, according to Ms. Sadik-Khan, including London, Paris and Bogotá. She said that in Bogotá one of the city’s main streets was closed to motor vehicles every Sunday.

The plan got a mixed reaction on Monday along the route.

“I think it’s a lovely idea,” said Allison Blinken, 65, a retiree who lives on Park Avenue at 66th Street. “Anything that makes the street more pedestrian-friendly.”

Several other residents of the well-appointed apartment buildings along the avenue voiced support, although some noted that neither they nor many of their neighbors would be around to enjoy the benefits or bear the annoyances, since the area virtually empties out on summer weekends as people head to the Hamptons or elsewhere.

Downtown, however, there was a fair amount of grumbling over the potential impact on business.

“He’s got to be crazy,” Pablo Urema, 49, a worker at a parking lot on Lafayette Street in SoHo, said of the mayor. “We do a lot of business every Saturday morning. No cars for the parking garage means no people for the businesses.”

Tran Harper, 44, the manager of Canal Lafayette Store, which sells Chinese teas and herbal products on Lafayette Street in Chinatown, was also displeased.

“It’s a big problem because my merchandise doesn’t fall from the sky,” Mr. Harper said. “How do I get it here? Saturday is the busiest day. We have a lot of deliveries on Saturday. Also a lot of customers park their cars in front and come in to buy.”

At the news conference Mr. Bloomberg responded with peevishness when asked about the potential for a negative reaction from business owners or residents. “I knew you were going to find something wrong with it,” he said to a reporter.

“Look, there will be minor inconveniences,” he said. “There’s minor inconveniences when it rains, when you have snow, inconveniences when it’s hot, when it’s cold, inconveniences when there are people on the streets, when there’s not.”

But the mayor predicted that most stores would see an increase in business and compared the initiative to his ban on smoking in bars and restaurants, which met with initial resistance but ended up being popular.

Ms. Sadik-Khan said that the Transportation Department had worked with the Police Department to design a route that would be less disruptive to traffic.

“We’ve been making very careful preparations on our traffic front to make sure that we’re dealing with the access issues that residents have, with the delivery issues that businesses have and also to make sure that we’ve got appropriate access for emergency vehicles,” she said.

Cyclists who were asked about the initiative on Monday were generally enthusiastic.

Eric Monasterio, 32, a painter who lives in Fort Greene, Brooklyn, said he regularly battled cars as he rode his bike in Manhattan.

“If it works out they could have a rotating system and have one street open every day,” Mr. Monasterio said.