An explosion in a crowded Manhattan Chelsea neighborhood on Saturday night injured 29 people hours before world leaders arrived for a high-level UN summit.

Authorities removed a suspicious device that was discovered just four blocks away from the explosion site. Mayor Bill de Blasio called the blast an "intentional act," but said there was no terrorist connection yet.

It is still unclear who is behind the blast, and what motivated it, before the UN summit, which will start on September 19 at the UNHQ in New York and host world leaders to discuss the refugee and migrant crisis across the world."Tonight, New York City experienced a very bad incident," de Blasio said at a news conference near the scene in Chelsea. "We have no credible and specific threat at this moment. "

De Blasio said the blast was "an intentional act" and tried to calm any fears among nervous New Yorkers, saying the explosion had no terrorist connection and wasn't related to a pipe bomb explosion earlier Saturday in New Jersey at a charity run.

The device, which injured 29 people, was removed with a robot and taken to the department firing range in the Bronx, officials said.

The second device that officers removed four blocks from the scene was a pressure cooker attached to wiring and a cellphone.

The source, speaking on condition of anonymity because the person was not authorized to speak about an ongoing investigation, said the device was found inside a plastic bag on West 27th Street.

The law enforcement official also said that the explosion appears to have come from a construction toolbox in front of a building. Photos from the scene show a twisted and crumpled black metal box.

The blast happened on West 23rd street, in front of a residence for the blind, near a major thoroughfare with many restaurants and a Trader Joe's supermarket. Witnesses say the explosion at about 8:30 p.m. blew out the windows of businesses and scattered debris in the area. Officials said no evacuations were necessary.