Temporary supports have been installed inside the Midtown Manhattan office building that was showing signs of collapse, Mayor Zohran Mamdani of New York said on Wednesday, and there has been no additional shifting since the structural damage was discovered a day earlier.

Construction crews hammered and welded through the night to shore up the 20th and 21st floors of 235 East 42nd Street, a 37-story building that is being converted into a housing complex with more than 1,600 apartments. Four new floors and a large vertical portion had been added onto the existing building in recent months.

“I can say right now the building is stable,” the city’s buildings commissioner, Ahmed Tigani, said late Tuesday. “We feel confident in the emergency plan we have now.” Mr. Tigani sought to reassure the public, even as he warned of tense days ahead in the area around the building, which was the former corporate headquarters of the pharmaceutical giant Pfizer.

Mr. Mamdani echoed Mr. Tigani’s comments at a news conference in Downtown Brooklyn on Wednesday, adding that the city would “continue to prioritize the safety of all that immediate area.”

Police officers blocked all traffic on East 42nd and 43rd Streets between Second and Third Avenues, and authorities said early Wednesday that traffic in that area remained restricted. Those who worked or lived in the area would have access, however, unless the buildings were under evacuation orders.

Officers with the New York Police Department’s Technical Assistance Response Unit flew a drone beside the building, close to the 21st floor, throughout the night. The video captured two workers in hard hats inside the building, inspecting the floor where support beams had buckled.

Fire officials had received reports on Tuesday morning about “a structural issue” in the building. Architects had called the project, scheduled to be completed in 2027, the largest of its kind in the city’s history.

Two support columns inside the building began buckling and several upper floors were sagging, the Fire Department said on Tuesday. The authorities initially created a “frozen zone” from 40th to 45th Streets between First and Third Avenues as they worked to stabilize the building. Although the so-called frozen zone has shrunk considerably, Mr. Tigani said that “the public should not engage with that area.”

The situation disrupted Midtown Manhattan, as construction workers and people in nearby buildings, including tourists and school students, were evacuated. There were no injuries, the Fire Department said.

Here’s what else to know:

Developer’s response: Nathan Berman, the founder of MetroLoft, the developer behind the project, said in an interview that there was never any danger that the building would collapse, calling the episode “a typical construction mishap.” A spokesman for the city’s Buildings Department said the structure was still being stabilized and an investigation was continuing.

Upper floors: Mr. Tigani said late Tuesday that officials were monitoring the building for any signs that it was unstable. Emergency shoring was being undertaken on the 20th and 21st floors of the building, Brad Hoylman-Sigal, the Manhattan borough president, said in a social media post late Tuesday.

Midtown construction: The project is part of a campaign to turn Midtown Manhattan’s empty office buildings into residential spaces to help address a housing shortage and revitalize the area.

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