A loud explosion has shaken the busy Chelsea neighborhood of Manhattan in New York City, injuring 29 people on Sep 17.

“Explosion happened at roughly 8.30pm on 23rd street between 6th and 7th avenues,” in Manhattan, police spokesman J. Peter Donald tweeted. “Several injured transported to area hospitals.”

Speaking to the press late Saturday evening following an explosion on 23rd Street in Manhattan’s Chelsea district, New York Mayor Bill de Blasio said that the “serious” incident had left a “significant” number of injuries.

“Tonight, New York City experienced a very bad incident,” de Blasio told reporters near the scene in Chelsea. “We have no credible and specific threat at this moment.”

De Blasio said the blast was an intentional act, but added that at the moment there was no evidence of a connection to terrorism or to a pipe bomb explosion in New Jersey earlier in the day.

New York Police Commissioner Jimmy O’Neill said the exact cause of the explosion – which left 29 people injured – had not yet been determined and that officers are investigating a second device found a few blocks away.

The New York Fire Department said that 24 people were taken to hospital. One person is reportedly in critical condition after suffering a puncture wound. The other injuries are said to be just scrapes and bruises.

An unnamed law enforcement official told The Associated Press that the second device appeared to be a pressure cooker wired to a mobile phone. The officer could not confirm whether the devices are related.

New York City Police advised motorists in the area that they should “expect extensive traffic delays and emergency personnel in the area of 23rd Street and 7th Avenue” due to police activity there. They also asked the public to avoid the area.

The blast is believed to have come from a large plastic trash bin on 23rd Street, a major east-west route in the fashionable downtown neighborhood of Chelsea. A gas explosion has been ruled out.

Earlier in the day the Semper Five charity run in the US state of New Jersey was called off after a pipe bomb exploded close to the route.

As a precaution, New York Police Department said it had increased security across some parts of the city.

The US presidential candidates, Hillary Clinton and Donald Trump, responded to news of the explosion.

“Boy we are living in a time – we better get very tough, folks,” said Trump. “It’s a terrible thing that’s going on in our world, in our country and we are going to get tough and smart and vigilant.”

Clinton stressed that it is was important to wait for more information on the explosion before making conclusions. “I think it is important to know the facts about any incidents like this,” the democratic candidate said. “That is why it is critical to support the first responders, the investigators who are looking into it, trying to determine what happened.”