Video report by ITV News Correspondent Geraint Vincent
Armed police and troops have been deployed across the UK in response to the terror threat being raised to critical - as the hunt for the Parsons Green bomber continues.
Prime Minister Theresa May announced the threat now stood at critical, meaning an attack is expected imminently.
A manhunt began after an improvised explosive device sent a fireball through a packed London Underground train carriage during the Friday morning rush-hour.
So-called Islamic State has claimed responsibility for the attack in south-west London which injured 29 people.
ITV News understands that Scotland Yard knows the identity of the attacker.
The country's top counter-terrorism officer has suggested that more than one person could have been involved in the plot.
Parsons Green station re-opened in the early hours of Saturday.
Mrs May said military personnel would replace police officers "on guard duties at certain protected sites which are not accessible to the public".
"The public will see more armed police on the transport network and on our streets, providing extra protection," she said.
"This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses."
The use of the military is believed to allow up to 5,000 troops to be deployed in support of the police.
Is raising the threat level to critical unusual?
Yes - though not unprecedented. It is the fourth time the assessment has been placed at the highest level for the past 11 years. It has only been at critical three times, in August 2006 and June 2007 and in May after the Manchester Arena bombing.
How has the threat changed over the years?
The level was first made public in 2006. Since then, it has most often been at severe, and not been lower than substantial (an attack is a strong possibility).
Who decides the terror threat level?
An organisation called the Joint Terrorism Analysis Centre. It was established in June 2003 and is based at MI5's London headquarters. JTAC comprises representatives from 16 government departments and agencies.
A number of factors may be taken into account, including available intelligence, terrorist capability, terrorist intentions and timescale.
Metropolitan Police Assistant Commissioner Mark Rowley said police were "chasing down suspects" over the attack.
ITV News understands that the Met know the identity of the attacker.
In a statement outside Scotland Yard, the UK's most senior anti-terror police officer said: "Somebody has planted an improvised explosive device on the Tube - we have to be open-minded at this stage about him and about potential associates."
Mr Rowley said detectives were only aware of one device and refused to be drawn on details of the suspects because of the "covert" nature of the operation.
"Detectives have spoken to tens of witnesses and we have taken a large number of calls to the anti-terror hotline from members of the public," he said.
"And indeed members of the public have sent in so far 77 images and videos they have taken at the scene and these have been sent into our appeal website and these are being assessed for evidential value.
"Meanwhile, the improvised explosive device on the train, the remnants of it have now been made safe and they've been taken away for specialist examination by forensic scientists.
"So whilst we chase down the suspects, as the public would expect, we are strengthening our policing resources on the streets of London and across the country whilst continuing the investigation.
Twenty-nine people are in hospital, most of whom suffered "flash burns", after the improvised explosive device went off at Parsons Green station in south-west London at around 8.20am.
So-called Islamic State claimed responsibility for the attack through its Amaq news agency, according to the US-based Site Intelligence.
Rita Katz, the director of Site, said IS claimed the bombing was the work of a "detachment" rather than simply a "soldier", which she said implied it was a coordinated attack.
The terror group has frequently declared that it was behind terrorist incidents in recent years but experts have warned their claims should be treated with caution.