Police hunting four bombing suspects have searched an address in the Harrow Road area of north-west London.
Residents reported hearing bangs, believed to be the firing of CS gas cannisters into a house, but police said no arrests were made.
One neighbour told BBC News 24 that armed officers had taken over her living room, telling her there may be a bomb in the flat opposite.
A second focus of the police operation appeared to be a nearby internet cafe.
In that incident, in Harrow Road, West Kilburn, BBC defence correspondent Paul Adams said locals had spoken of dramatic scenes where two women, described as being of Middle Eastern appearance, and a young boy were detained by armed officers and forced to the ground.
One or more were led away by police, he was told.
He said there had also been reports of the women and a boy being taken away in an ambulance, but it was not clear if anyone had been injured.
Adams added the situation was "clear as mud".
Armed officers in body armour, several police vehicles and an ambulance were at the scene. A stretch of road was sealed off and residents told to stay in their homes, but the cordon has now been lifted.
Mohammed Kalam, 29, said he had been on a bus when he saw police surrounding two men outside an internet cafe.
He later saw a young boy and a woman, both with their hands on the wall of the cafe, he said.
The operation followed the shooting dead of a suspect in Stockwell
"The boy was wearing a red t-shirt but police then made him put on a white outfit," he said.
"I heard police say: 'Put your hands against the wall. Don't panic'."
He said he then saw the people being detained getting into an ambulance.
A student, who gave his name as Ahmed, said he was coming out of a barber's shop on when he saw someone coming out of the internet cafe with "what looked like gas masks".
In nearby Portnall Road, off Harrow Road, a house was raided by officers, who reportedly first sent in a remote-controlled bomb disposal unit before firing CS gas in the windows and basement.
Venetia Elphick, 40 - who said her flat was used as a police base for the raid - told the BBC that armed officers knocked on her door and asked to come in.
She said: "They took over my sitting room and told me I had to sit at the back of the flat.
"I was absolutely terrified because they let me know that there was a bomb, possibly, in the house opposite, on the other side of the street.
"They sat at each window with their machine guns and they told me to leave the sitting room, so I couldn't see face to face what was happening."
She said she heard shots - although did not know how many - and the sound of the "hissing of gas".
The officers were in her flat for around an hour and a half, she said.
"They said it was to do with the bombs yesterday."
Another resident Kenneth Mulligan also said police used his house during the operation.
He said the officers were pointing the guns out the window towards the house across the road. Police sent a machine into the house across the road, he said.
"I heard five bangs and then the sixth bang was a big one. I don't think it was a bomb, I think they were firing a canister of gas. The police had gas masks on."
In relation to the house raid in Portnall Rd, off Harrow Road, resident Patricia Osbourne told BBC News 24 she had heard six sounds that she believed were gunshots.
"We could quite clearly hear that there were shots being fired," she said. We could hear it but we couldn't see it."
Another one witness told the BBC he saw remote-controlled trucks that he thought were used by bomb disposal units.
Huse Monfaradi, 31, who lives in Portnall Road, off Harrow Road, told BBC News 24: "From my window I can see 20 doors down what looks like a bomb disposal-type vehicle. It's armoured and there are several armed officers around it.
"The police ran up my road telling everyone to get inside their houses. "Anyone who even puts their head out, they scream 'get inside your house'."