Three men have been arrested under the Terrorism Act after a man was killed in a car explosion outside Liverpool Women's Hospital.
A taxi carrying one passenger pulled up just before 11:00 GMT on Remembrance Sunday, as a national two minutes' silence was due to begin, and exploded.
The passenger was declared dead at the scene and is yet to be formally identified.
The male driver was injured and is in hospital in a stable condition.
Detectives from Counter Terrorism Police North West said three men - aged 29, 26 and 21 - were detained in the Kensington area of the city.
They said they were continuing to keep an open mind about the cause of the blast and were working with Merseyside Police as the investigation continued "at pace". The security service, MI5, is also assisting.
Armed officers have carried out raids on properties on Rutland Avenue near Sefton Park, and around Sutcliffe Street and Boaler Street in Kensington.
Specialist officers remain outside a house on Rutland Avenue, in an operation police say is linked to the explosion.
Police negotiators are on the scene, a fire crew is on standby, and a number of residents have been evacuated.
A large cordon, guarded by uniformed officers, remains in place.
Carl Bessant, whose partner had just had a baby, was inside the hospital at the time of the blast.
"My partner is really shook up to be honest," he said.
"We were so close and she was feeding the baby when it happened. We heard a loud bang and looked out of the window.
"We saw the car on fire and someone jump out... screaming, and there was someone inside the car.
"The hospital shut down, no-one in or out, so they said, but people were using the back entrance."
Prime Minister Boris Johnson said on Twitter: "My thoughts are with all those affected by the awful incident in Liverpool today.
"I want to thank the emergency services for their quick response and professionalism, and the police for their ongoing work on the investigation."
Home Secretary Priti Patel also tweeted she was "being kept regularly updated on the awful incident".
At the scene
By Jim Clarke, BBC North West Tonight deputy editor
The first raid happened at about 13:00 GMT on Rutland Avenue in the Sefton Park area of the city. It's about 0.75 miles from the hospital. It's an area of the city with large Victorian houses.
The house that was raided had been split up into flats. Local residents said they had not seen anybody coming or going from the property for some weeks - they thought it was unoccupied. The street remains cordoned off and some residents have been evacuated.
Police were also seen at Boaler Street in the Kensington area of the city. It's about a mile away from the hospital.
Eyewitnesses on the street said armed police arrived at about 14:00 and raided a house. They said they saw men being arrested and handcuffed. They were led by armed officers into a car. That street also remains cordoned off.
Chief Constable Serena Kennedy, of Merseyside Police, sought to reassure the public, saying events of this nature were very rare and that there would be an increased and visible police presence on the streets in the coming days.
A bomb disposal unit and fire engines attended the scene earlier, which remains cordoned off. Road closures are also in place.
Police were called at 10:59 to reports of a car explosion.
Phil Garrigan, chief fire officer of Merseyside Fire and Rescue Service, said the car fire had been "fully developed" when crews arrived shortly after 11:00.
"The operational crews extinguished the fire rapidly but... there was one fatality," he said.
"Another individual had left the vehicle prior to the fire developing to the extent that it did. Our thoughts are with them and the families of those involved."
Nick Aldworth, a former counter terror co-ordinator, said investigators would be looking to ascertain what happened inside the vehicle.
"They'll be looking at what sort of damage has been caused, trying to get an assessment of what might have caused that blast," he told BBC Radio 4's Today programme.
"From what I've seen there is very little blast damage - a lot of fire damage but very little blast damage.
"So whatever was in that vehicle was either a low yield or didn't work properly or was possibly an incendiary."
Mr Aldworth explained it was common for investigations to be linked to terrorism quickly as it provides investigators with more resources from an earlier stage.
He added that it was entirely appropriate to involve MI5 to understand if it holds relevant information.
Liverpool Women's Hospital said visiting access had been restricted until further notice and patients had been diverted to other hospitals where possible.
"We are reviewing our patient activity for the next 24-48 hours and patients should wait to be contacted for updates about any planned appointments or other attendance at the hospital," chief executive Kathryn Thomson said.
Liverpool Women's Hospital receives about 50,000 patients annually and is the largest hospital in its specialism in Europe.
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