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 police.
A few hours later, the authorities found and removed what they described as a second explosive device four blocks away, raising the possibility that two bombs had been planted in the heart of the city.
Mayor Bill de Blasio called the explosion — which occurred about 8:30 p.m. on West 23rd Street — “an intentional act” but initially said there was no connection to terrorism and no immediate claim of responsibility.
Police officers swarmed Chelsea’s streets after 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.”
As the authorities sought to identify what had caused the explosion, they described the second device as a pressure cooker resembling the one used in the deadly Boston Marathon bombings in 2013, according to a police official, who spoke on the condition of anonymity to discuss a continuing investigation.
It was unclear whether the blast on West 23rd Street had been caused by the same type of explosive.
In the immediate aftermath, the police shut down a swath of Manhattan south of Midtown. The area from 14th Street to 32nd Street was closed to traffic between Fifth and Eighth Avenues. But by 7 a.m., only 23rd Street remained closed.
A grim Mr. de Blasio, speaking at a news conference at the scene around 11:15 p.m., 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 started inside a sidewalk Dumpster near the Avenue of the Americas. Images of a twisted Dumpster in the middle of West 23rd Street quickly proliferated on Twitter.
The impact shattered windows, damaged cars and sent crowds running from the scene at an hour when Chelsea, always a popular destination, was filled with residents and tourists.
“It was the biggest blast I ever would imagine, lights flashing, glass shattering,” said a women who was injured in the explosion.
The force of the explosion, she said, flung her into the air.
“It happened so fast I was thrown up and landed down, I didn’t know where it had come from,” said the woman, who would give only her first name, Helena, as she hobbled out of Bellevue Hospital Center about 4 a.m. after she was treated for injuries to her eye and legs. “I realized there was blood streaming down my face, and I couldn’t see out of my eye.”
Luke McConnell, who was visiting from Colorado, was headed toward a restaurant on West 27th Street when the blast occurred. “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 the Avenue of the Americas with friends.
“We felt the shock waves go through our bodies,” he said.
It was a startling scene, full of dark possibilities, for a city that endured the attacks of Sept. 11, 2001, but has so far been spared the kind of mayhem that has terrorized city after city around the world in the 15 years since.
The closest New York has come to an attack was in 2010, when the police found a crude car bomb of propane, gasoline and fireworks inside a sport utility vehicle in Times Square. Although the device had apparently started to detonate, there was no explosion.
On Saturday night in Chelsea, the device found on West 27th Street also caused no harm.
Images shared on social media and confirmed as authentic by a senior police official showed a silver-colored piece of cookware with wires and a cellphone attached.
The official said the Police Department’s bomb squad was taking the device to a department facility in the Bronx, where robots would inspect it.
Around 2:25 a.m., a Police Department truck towing a spherical chamber, which contained the device, headed east on West 27th Street and turned up the Avenue of the Americas. Several police officers who had spent the evening on alert were visibly relieved, as one by one they let the few residents who had been waiting all night beside the caution tape return home.
It was a cool Saturday night, and the businesses along West 23rd Street, the busiest east-west thoroughfare in Chelsea, were teeming with customers.
The blast seemed to shake the entire block, smashing windows in a five-story brownstone building and sending debris into the street, a law enforcement official said.
The sidewalk where the explosion occurred is in front of a nondescript building wedged between a church and an apartment building.
Video captured before the explosion shows a man crossing “the street in the direction of where the device was found,” the same official said. But no video had yet been obtained clearly showing anyone placing the device in the spot where it detonated.
“We don’t understand the target or the significance of it,” the police official said. “It’s by a pile of Dumpsters on a random sidewalk.”
Marcello Begu, 58, was spinning pizzas at the nearby Ciao Bella Napoli restaurant when he heard the blast.
“I’ve never heard a noise like that in my life,” he said. “The ground was shaking. I was scared to go outside.”
In Washington, the White House issued a brief statement saying that President Obama had been briefed on the developing situation in New York.
Both the Democratic and Republican presidential candidates broke from their campaign routines to address the issue.
Donald J. Trump, in Colorado Springs, rushed to describe the explosion as a bomb well 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.
She seemed to scold Mr. Trump for his quick assessment.
“I think it’s always wiser to wait until you have information before making conclusions,” Mrs. Clinton 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.