The man had been pacing with a loaded shotgun and police had been negotiating with him in an effort to try to drop the gun. He was described as a man in his 60s.
Things started unfolding about 10:40 a.m. on Thursday. Police had taped off an area along 42nd St. and 1st Ave.
The man did not appear to try to breach the security perimeter of the UN complex but it was put on lockdown and employees were told to shelter in place. A UN spokesperson said that operations had returned to normal after the man was peacefully taken into custody.
The man said he had papers he wanted to be delivered to the UN. Officers agreed to give the papers to UN officials and the man then agreed to surrender. Police said they were some sort of medical records.
He was taken to the hospital for observation. The man traveled from Florida and had no prior arrest record.
Police searched the man's truck and a room at the Millennium Hotel he had stayed on Wednesday night.
His name was not immediately released.
There was a mass police response across the entire area. The sounds of police sirens filled the air on the East Side of Manhattan due to the situation.
The NYPD counterterrorism team arrived on the scene just before 12:30 p.m.
Images from the scene appeared to show the man holding a shotgun to his chin at one point. The video also showed NYPD officers in tactical gear and long guns.
Several ambulances were also on standby at the scene.
The NYPD asked people to avoid the area. 42nd St. between 1st and 2nd Ave. was closed along with 1st Ave. between 39th and 47th Streets. Traffic on the FDR Drive was also impacted by the situation. The road travels under the UN complex.
The United Nations complex consists of four main buildings on the shore of the East River in Manhattan.