"There is no specific and credible threat to New York City from any terror organization," de Blasio said late Saturday at the news conference.
"We believe at this point in time this was an intentional act. I want to assure all New Yorkers that the NYPD and ... agencies are at full alert," he said.
More on the blast in New York City:
- NYC blast: What we know now
- Here's what people in the Chelsea area saw
- Raw video from the scene in New York City
- Chelsea is one of New York City's premier neighborhoods
- Trump cites 'bomb' in NYC blast
- Images from the scene
Anyone with any information regarding the explosion was asked to call 1-800-577-TIPS.
The cause of the explosion had not been determined, but there was no sign that it was caused by a natural gas explosion, New York Police Commissioner James O'Neill said at the news conference.
"The investigation is active and at this time an extensive search is being conducted,” O'Neill said. "The area around the explosion site is being treated as a crime scene," O'Neill said.
Based on what was known, there was also no connection to the earlier incident Saturday in Seaside Heights, N.J., where a pipe bomb exploded near a Marine charity run, de Blasio said. In that instance the device was placed in a garbage can. No injuries were reported. Ramon Lopez was at West 23rd and Sixth Avenue when the explosion happened. “It felt like a building was coming down,” said the 48-year-old East Harlem resident.
He ran about a half-block away then turned around and ran back to the scene to help people. He spotted a woman with a metal fragment in her eye saying, “I can’t see. I can’t see.” Lopez took her by the arm to an ambulance that had just arrived on the scene.
Lopez saw other victims bleeding from small spherical fragments and metal shards.
“I was telling (the victims) it was minor, but it was major,” he said. “If I told them it was major they would collapse.”
Klaas Claes, co-owner of BXL Zoute, a Belgian restaurant on West 22nd near the blast scene, said the explosion sounded like the rear gate slamming on a large dump truck, "only ten times louder."
"You could feel it in your body, it was very powerful'" said Claes.
He and some restaurant customers ran outside, but saw no immediate signs of fire or smoke, Claes said.
Police and fire department responders arrived at the scene moments later, he said.
Manhattan Borough President Gale Brewer said the Chelsea explosion was not in a dumpster but next to one in the street.
Blast at 8:30 p.m.
The explosion came just after 8:30 p.m. ET at 133 W. 23rd St., between Sixth and Seventh Avenues. None of the injuries appeared to be life-threatening, the New York City Fire Department said.
The cause of the explosion has not been determined. "The investigation is active and at this time an extensive search is being conducted,” New York Police Commissioner James O'Neill said at the press conference.
"The area around the explosion site is being treated as a crime scene," O'Neill said.
Several injured were transported to area hospitals, assistant commissioner for communication with the New York City Police Department, J. Peter Donald, tweeted.
The explosion area occurred a block away from Eataly, a popular Italian market and eatery. The neighborhood has many late-night and after-hours music clubs. At 11 p.m., New York police and news helicopters hovered over the scene beneath a full harvest moon.
“Whatever the cause, New Yorkers will not be intimidated, they will not let anyone change who we are,” deBlasio said.
Tweets from the area describe "one loud long boom."
The New York Police Counter-Terrorism unit responded to the blast and transit service in the area was shut down, according to the New York Fire Department.
The White House said President Obama was apprised of the explosion.
News was just trickling out of some kind of explosion in New York City on Saturday night when Donald Trump said a "bomb" was behind the the blast in remarks at a campaign appearance at Colorado Springs.
“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 but, boy we are really in a time – we better get very tough, folks. We better get very tough. We’ll find out."
Democratic presidential nominee Hillary Clinton was more cautious, saying she had been briefed “about the bombings in New York and New Jersey and the attack in Minnesota.”
“We have to let this investigation unfold,” she said. She added that the nation needs to support its first responders and “pray for the victims.”