Police have raided a block of flats close to Manchester Piccadilly Station following Monday's attack in the city.
Residents described how armed police and men clad in balaclavas stormed the Granby House building on Granby Row, in central Manchester.
The railway was briefly closed during the raid, police said.
A Greater Manchester Police spokeswoman said: "Officers have this afternoon carried out a search at an address in Manchester City Centre as part of the investigation into the horrific incident at Manchester Arena.
"That search is ongoing.
"In order to do this safely we briefly closed a railway line, but it has now been reopened."
Louise Bolotin, a freelance journalist who lives in the building, described how the fire alarm went off before police stormed the building and broke into a flat on the third floor.
"I came running down the front staircase, six floors, and got to the front entrance and was met by a police officer in full-on tactical helmet, face mask, machine gun. I was expecting to see the fire brigade."
She added: "I said 'what's happening and he just said out'."
Marcus Murray, 27, lives on the first floor of the building and was looking on as officers went in and out.
"I wanted to come back into my flat but there were unmarked cars.
"I got to the front door and there was a guy with a gun who said I can't go in," he said.
Mr Murray, a software engineer, added: "I feel a bit uneasy. Going to sleep is going to be a bit difficult. It just feels unsafe.
"You just don't expect it do you? It's very strange," he said.
Mr Murray said it is common for people to come and stay in flats in the building just for the weekend.
"It's usually a very quiet street," he said, adding: "I felt safe before, but not now."
Ms Bolotin said there were 62 flats in the building with a "transient population".
Around half of the apartments are owner-occupied while the rest are rented out, she added.
Ms Bolotin described the raid as "absolutely massive".
She said: "I saw down there about four armed officers coming out from the back of the building.
"Again, helmets, face masks, machine guns and getting into two unmarked cars and driving away.
"After that about half a dozen army came out in camouflage, weapons.
"There's still a lot of armed officers in the building, some have left. We've had the fire brigade here, at least one fire officer is still in the building."
She added: "As far as I know no one has been arrested and brought out of the building."
Resident Lynne Deakin, 53, who lives on the ground floor of the flats, said she was "not at all surprised" by the raid.
"I've often said, if I wanted to take out Piccadilly Station, because the line is right at the back, I would put a bomb in that building," she said.
"I'd park a car underneath. It's been so obvious to me for years."
She told how she turned off the fire alarm and headed upstairs, where she saw five or six "army" men outside the raided flat.
"I'd gone up the stairs ... I was at one end of the corridor and they were at the other through a fire door," she said.
"So one of the guys came out and he said 'get downstairs'.
"It was the proper guys that I've seen on the TV with the balaclavas and the big helmets."
Ms Deakin said while she knew her neighbours she did not know many people in the building.
She said she believed the raided flat had been rented out on property site Airbnb.
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