A powerful explosion caused by what the authorities believe was a homemade bomb injured at least 29 people on a crowded sidewalk in the bustling Chelsea neighborhood of Manhattan on Saturday night, according to the authorities, who later found and removed what they described as a second explosive device four blocks away.
Mayor Bill de Blasio called the explosion “an intentional act” but initially said there was no connection to terrorism. He cautioned that the authorities had just begun their investigation into the blast, which reverberated across a city scarred by terrorism and vigilant about threats just days after the 15th anniversary of the Sept. 11 attacks.
“Whatever the cause,” Mr. de Blasio said, “New Yorkers will not be intimidated.”
The explosion, which occurred around 8:30 p.m., prompted the shutdown of a wide swath of Manhattan south of Midtown. The Police Department said at 1:15 a.m. Sunday that 14th Street to 32nd Street was closed to traffic between Fifth and Eighth Avenues until further notice.
A grim Mr. de Blasio, speaking at a news conference at the scene, said “injuries are significant.” But for the moment, he said, none of them were life-threatening.
Many of the injuries were caused by shrapnel from the explosion, which witnesses said seemed to have come from a sidewalk Dumpster on West 23rd Street near the Avenue of Americas. Images of a twisted Dumpster in the middle of 23rd Street quickly proliferated on Twitter.
The explosion shattered windows, damaged cars and sent crowds running from the scene.
“I heard a big boom,” said Luke McConnell, who was visiting from Colorado and had been headed toward a restaurant on West 27th Street. “I felt it, like a concussive wave, heading towards me.”
“Then there was a cloud of white smoke that came from the left side of 23rd Street near Sixth,” he said. “There was no fire, just smoke.”
Witnesses said they could feel the explosion from several blocks away. Daniel Yount, 34, said he was standing on the roof of a building at 25th Street and Avenue of the Americas with friends.
“We felt the shock waves go through our bodies,” he said.
A law enforcement official said investigators were trying to figure out who was behind the explosion and what the motivations might have been.
“It likely came from an improvised device,” said one city official who spoke on the condition of anonymity to discuss a continuing investigation. “We don’t understand the target or the significance of it. It’s by a pile of Dumpsters on a random sidewalk.”
The other device, on West 27th Street, was described as resembling a pressure cooker like the one used in the Boston Marathon bombings in 2013, the official said.
A senior police official confirmed that photographs of the device shared on social media, which showed a silver piece of cookware with wires and a cellphone attached, were authentic. The official said the Police Department’s bomb squad was taking the device to a department facility, where robots would inspect it.
The official said investigators “don’t have any suspects” and were not questioning anyone.
The official said that video captured before the explosion shows a man crossing “the street in the direction of where the device was found.” But no video had yet been obtained clearly showing anyone placing the device in the spot where it detonated.
Pressed on who might be responsible for the incident, James P. O’Neill, the police commissioner, said no suspect was in custody. “We are still in the process of trying to figure that out,” he said.
Mr. O’Neill said the authorities had collected video of the explosion but asked the public for any footage they might have, as well as eyewitness accounts and tips
The blast shattered windows in a five-story brownstone building and sent debris into the street, a law enforcement official said. The building is between a church on its eastern side and an apartment building under renovation on its western side.
The police commissioner said that residents along 23rd Street were being allowed back in their homes and that there was no need for mandatory evacuations.
The explosion also disrupted subway service on the F and E lines, the Metropolitan Transportation Authority said on Twitter.
In Washington, the White House issued a brief statement saying that President Obama had been briefed on the developing situation in New York.
“The president has been apprised of the explosion in New York City, the cause of which remains under investigation,” the statement said. “The president will be updated as additional information becomes available.”
State officials said they were coordinating their work with federal and New York City authorities.
“We are closely monitoring the situation and urge New Yorkers to, as always, remain calm and vigilant,” Gov. Andrew M. Cuomo said in a statement.
“We are in close contact with health providers to monitor the extent of the injuries and will provide information as it becomes available,” he added.
Both presidential candidates broke from campaign routines to address the issue.
The Republican presidential nominee, Donald J. Trump, weighed in on the news while campaigning in Colorado Springs on Saturday night. Mr. Trump made the remarks about 45 minutes after the explosion was reported, before the authorities had made any determinations about what had happened and while the situation was still in flux.
“I must tell you that just before I got off the plane, a bomb went off in New York and nobody knows exactly what’s going on,” he said. “But boy, we are living in a time — we better get very tough, folks.”
The Democratic nominee, Hillary Clinton, was informed of the episode after she gave a speech at the Congressional Black Caucus Foundation’s annual awards dinner, her campaign said.
Officials said the New York explosion was not connected to a blast that happened 11 hours before when an improvised device exploded in a garbage can near the course of a charity race that was about to start in a small town on the Jersey Shore. That device went off around 9:30 a.m. near the boardwalk in Seaside Park, N.J., according to the Ocean County sheriff, Michael G. Mastronardy.
There were no injuries. The race, the Seaside Semper Five, a five-kilometer run and charity event along the waterfront that raises money for members of the United States Marine Corps and their families, was canceled.