Police say they are investigating a “network” over Monday night’s attack at Manchester Arena, in which 22 people were killed.
An off-duty policewoman was confirmed by police to be among the victims.
Salman Abedi blew himself up outside an Ariana Grande concert. Four people have been arrested, including the bomber’s 23-year-old brother.
The UK terror threat level is now up to its highest level of “critical”, meaning more attacks may be imminent.
Military personnel are being deployed to protect key sites.
The Palace of Westminster has been closed to the public following police advice, and will not re-open until further notice, its website said.
Home Secretary Amber Rudd said: “[Monday’s attack] was more sophisticated than some of the attacks we’ve seen before, and it seems likely – possible – that he wasn’t doing this on his own.”
BBC security correspondent Frank Gardner said that the bomber is thought to have been a “mule”, using a device built by someone else.
Four people have been arrested since Monday night, including Abedi’s older brother Ismael who was detained in Chorlton, south Manchester, on Tuesday.
Chief Constable Ian Hopkins said: “I think it’s very clear that this is a network that we are investigating.
“And as I’ve said, it continues at a pace. There’s extensive investigations going on and activity taking place across Greater Manchester, as we speak,” he told reporters on Wednesday afternoon.
Police also raided a block of flats near Manchester Piccadilly station in the city centre, requiring them to carry out a controlled explosion and briefly close the railway line.
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