Earl Wilson/The New York Times

The cleanup of the Lower Manhattan park that has been occupied by protesters for nearly a month was postponed Friday shortly before it was supposed to begin, averting a feared showdown between the police and demonstrators, who had vowed to resist any efforts to evict them from their encampment.

But sporadic clashes between protesters and the police erupted anyway when demonstrators started marching through the winding streets of the financial district after learning that the cleanup had been called off. At least 15 were arrested, the police say.

Mayor Michael R. Bloomberg, in a radio interview on Friday morning, sought to make it clear that the decision to postpone the cleanup was made by Brookfield Properties, the owner of Zuccotti Park.

He said the company made its decision after receiving a flurry of threatening phone calls from elected officials, though he said he did not know who those officials were or what they had specifically told Brookfield. It was also unclear how closely Brookfield and the Bloomberg administration had consulted before the decision on the clean-up operation was made.

“Yesterday, as of 8 o’clock at night, they were going ahead to do it, but, as of midnight, they called and said they wanted to postpone the cleaning operations,” Mr. Bloomberg said on his radio program on WOR-AM (710).

State Senator Daniel L. Squadron said on Friday that he had several conversations on Thursday night with Richard Clark, the chief executive of Brookfield, and “other stakeholders,” urging Brookfield to delay the cleanup. “Brookfield Properties made the right decision in postponing its scheduled cleanup of Zuccotti Park,” he said.

The announcement on the cleanup was made by the Bloomberg administration around 6:20 a.m., about 40 minutes before workers were scheduled to enter the park, which has been the home base for the Occupy Wall Street demonstrators, angered by what they see as an unfair and corrupt financial system.

“Late last night, we received notice from the owners of Zuccotti Park — Brookfield Properties — that they are postponing their scheduled cleaning of the park, and for the time being withdrawing their request from earlier in the week for police assistance during their cleaning operation,” Deputy Mayor Caswell F. Holloway said in a statement.

“Brookfield believes they can work out an arrangement with the protesters that will ensure the park remains clean, safe, available for public use,” Mr. Holloway said, “and that the situation is respectful of residents and businesses downtown.”

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As news that the cleanup had been called off rippled through Zuccotti Park, cheers erupted among demonstrators who had been preparing for a possible confrontation.

“I did not come here to look for a fight,” said Steve Sachs of Hightstown, N.J. “I’ve never been in a fight in my life. I’ve never been arrested. But I was ready to be arrested over this.”

But clashes between the police and protesters flared on various streets in the financial district.

About 7:40 a.m., a man was seen being led away in handcuffs on Broadway. Moments later, a woman who said she was his girlfriend identified him as Michael Rivas.

Shortly afterward, at Maiden and Water Streets, police officers were seen taking four people into custody, placing them into a police wagon. One of those men appeared have a gash on his forehead and blood running down his face.

At one point, it appeared that as officers tried to keep the crowd on the sidewalk, a bag of garbage was hurled from the crowd and hit one officer. That prompted that officer and another to wade into the crowd and apprehend a man.

The crowds marched in roadways, accompanied or pursued by officers on foot or riding scooters.

Near the corner of Beaver and Broad Streets, officers wearing helmets leaped from scooters, tackled a man to the ground and placed him in handcuffs. At the intersection of William and Wall Streets, officers stood behind metal barricades as protesters filled the street in front of them. Some protesters waved mops and brooms that had been used earlier to clean Zuccotti Park.

Near Broadway and Exchange Place, officers drove scooters into a crowd of marchers.

One man, a legal observer for the National Lawyers guild, had his leg pinned under a police motorcycle. Video appears to show an officer on a motorcycle, after running over the man’s leg, leaving his motorcycle parked on the man’s leg to go off to pursue protesters while the man writhes in pain.

Another video appears to show a police commander in a white shirt punching a demonstrator, though it does not show what came before.

Over the course of the morning, at least 15 people were arrested, including protesters who knocked over a police scooter, overturned trash cans, hurled bottles and sat in the street blocking traffic, said Paul J. Browne, the head police spokesman.

Brookfield’s abrupt decision to call off the cleanup seemed to frustrate Mr. Bloomberg. He said that if Brookfield decided that it did still want to clean the park, it would place the city in a more difficult situation.

“From our point of view,” Mr. Bloomberg said on the radio, “it will be a little harder, I think, at that point in time to provide police protection, but we have the greatest police department in the world and we will do what is necessary.”

He also said Brookfield had come under intense pressure on Thursday night from local officials not to try to clean the park.

“My understanding is that Brookfield got lots of calls from many elected officials threatening them and saying, ‘If you don’t stop this, we’ll make your life more difficult,’ ” Mr. Bloomberg said. “If those elected officials would spend half as much time trying to promote the city and get jobs to come here, we would go a long ways toward answering the concerns of the protesters.”

Hundreds of people had gathered overnight in Zuccotti Park in anticipation of what might happen on Friday, swelling the crowd to much larger numbers than have typically been encamped in the park, while others continued cleaning the park, which Brookfield complained had become filthy and a potential health threat.

Around 5 a.m., a collection of mops and brooms tood in a plastic bin on Liberty Street. Nearby were 27 buckets of soapy water. A woman handed out white rubber gloves to more than a dozen people. They walked to the west end of the park, at Trinity Place, and announced that they were going to begin a sweep, picking up and discarding objects that did not belong to anyone.

“This place is extremely important,” said Kyle Christopher, 27, a photographer from Buffalo, who has been part of the protests since their first week.

Patrick Bruner, a spokesman for Occupy Wall Street, said that on Thursday night protesters tried to deliver a petition with more than 100,000 signatures to City Hall, calling upon Mr. Bloomberg to allow the occupation to continue.

On Thursday night, the A.F.L.-C.I.O. sent a message to members asking them to show up at the park early Friday morning to support the protesters.

By 6 a.m., the crowd had swelled to more than a thousand.

Andy Friedman, 38, of Park Slope, Brooklyn, said he had come to the park in response to the union’s call for solidarity.

“For the past 30 years, banks in this country have been making out like bandits,” said Mr. Friedman, who works for the service employees’ union, S.E.I.U. “And the rest of us are going backwards.”

On Wednesday, Mr. Bloomberg announced that the park would be cleaned.

In a letter to the police commissioner, Raymond W. Kelly, dated Tuesday, Mr. Clark, Brookfield’s chief executive, wrote that conditions in the park had reached “unsafe” levels, and he reiterated his complaint that the encampment violated the law.

Overflowing garbage cans attract rodents, he wrote, gas-fired generators pose a fire hazard, bad smells abound, the lack of toilets are making things worse and complaints are mounting from disgruntled people who live and work nearby.

“In light of this and the ongoing trespassing of the protesters,” Mr. Clark wrote, “we are again requesting the assistance of the New York City Police Department to help clear the park.”

The protesters feared that Mr. Bloomberg’s announcement that the park would be cleaned was a prelude to their being banned permanently. An appeal quickly went out on Facebook and other sites calling for brooms, mops and various cleaning supplies as well as volunteers willing to donate elbow grease. After cleaning the place themselves, the protesters planned to form a human chain around the park to try to keep police officers from entering. Supporters had been urged to go to the park at 6 a.m. Friday “to defend the occupation from eviction.”

Clashes between the police and protesters have occurred several times since protesters began camping in the park on Sept. 17. Scores of protesters were arrested during a march to Union Square on Sept. 24, and several women were pepper-sprayed by a high-ranking police officer, in an episode that is being investigated by the Manhattan district attorney’s office.

On Oct. 1, about 700 protesters were arrested while trying to march across the Brooklyn Bridge. And 28 people were arrested after an Oct. 6 march in Lower Manhattan, which attracted thousands and drew the backing of large labor unions.

Triggered by a call to action from Adbusters, a Canadian magazine, Occupy Wall Street began as a protest against income inequality and what demonstrators portray as corporate greed, drawing several hundred people to Lower Manhattan but barely registering a blip on the radar of mainstream news media. Participants declared themselves the part of the “99 percent” to highlight their claim that 1 percent of Americans control much of the country’s wealth.

News of the protest steadily grew, fanned by coverage of pepper-spraying episodes, the mass arrests on the bridge and the eruption of similar protests nationwide and around the world. Its support base widened. Celebrities visited the protesters, among them Susan Sarandon and Kanye West. Unions, politicians and academics lent their support. The protest was taken up by more and more people, angry over chronic joblessness, rising debt and what they view as a grossly imbalanced economic system.

Al Baker, Rob Harris and Kate Taylor contributed reporting.