A year on from the Westminster attack, the MP who tried to save the life of a stabbed policeman has recalled the "eerie" silence after the officer died.
Defence minister Tobias Ellwood, who gave mouth-to-mouth to PC Keith Palmer, said he returned home to see his son in tears asking why someone had been killed and why his Dad had helped.
"All I could offer was there are some bad people in the world, but… it's the good people that win," he said.
Five people were killed on 22 March.
Khalid Masood drove his car at people on Westminster Bridge before stabbing PC Palmer in the Palace of Westminster.
Three people were killed outright on the bridge and one died later in hospital, but BBC's Newsnight has learned it could have been worse.
Security sources told the programme that barriers outside Parliament saved 25-30 lives on the day of the attack, as they forced Masood's car off the pavement and onto the road.
At the end of the barriers he cut back into the pavement and crashed in New Palace Yard before attacking PC Palmer.
Newsnight's investigation found Masood was trying to move to Saudi Arabia in May 2016, nearly a year before the attack. However, his work visa was turned down.
Meanwhile, police investigating Masood's background suspected him of radicalising another man and giving him religious instructions – but the man was arrested and released without charge.
The police and MI5 concluded that Masood had acted alone.
Mr Ellwood, who is medically trained, said he was one of a number of people that stepped forward that day.
"Eventually the doctor said ok, I think we're going to have to call it. I remember looking at him and saying, 'you're going to have to tell me to stop, sir, because otherwise I'm going to keep doing this'," said the MP, whose brother died in the 2002 Bali bombing terror attack.
"I do recall the silence. It was very eerie. Not a single movement of traffic, not a horn, not anybody speaking, no shouts, nothing whatsoever… I was then left there with a couple of the original policeman, who by this time were very, very upset because it was their colleague."
Events have been held to remember 14 people killed in four London terror attacks in 2017.
MPs observed a minute's silence in the Commons to remember the dead, while Prime Minister Theresa May has laid a floral tribute outside parliament.
Culture Secretary Matt Hancock said those who had lost their lives defending democracy "will not be forgotten".
Speaker John Bercow, the Archbishop of Canterbury and the head of the Metropolitan Police, Cressida Dick, were among those to attend a vigil of remembrance at Westminster Hall.
"A year ago today, on this estate and on Westminster bridge, we were visited by what I regard as evil," said Speaker's Chaplain Rev Rose Hudson-Wilkin, who led the service.
The message #LondonUnited will be projected in four locations "as an act of solidarity".
London Bridge, Finsbury Park Mosque, Parsons Green underground station, and the Houses of Parliament will have the phrase projected on them overnight.
The Mayor of London Sadiq Khan said Londoners stood together, "united against terrorism and in hope for the future".
The mayor's office has also organised a digital book of condolence that the public can send messages of solidarity to.
The book will become part of a 3D installation in City Hall that will be open to the public until 19 June – the anniversary of the Finsbury Park attack.
A year of terror in London
- 22 March: Five people are killed in Westminster after Khalid Masood mounted the pavement in his 4×4 on Westminster Bridge and ploughed into pedestrians. On-duty police officer PC Keith Palmer is stabbed to death, Masood is shot by armed officers
- 3 June: Eight people were killed after three Islamic extremists drove a van into pedestrians on London Bridge, and then launched a knife attack in Borough Market. All three attackers were shot dead by officers
- 19 June: Darren Osborne drove a van into a group of worshippers leaving Finsbury Park Mosque, killing 51-year-old Makram Ali and injuring nine others. Osborne was later sentenced to life in prison
- 15 September: A homemade bomb partially explodes on a District Line train at Parsons Green tube station, injuring about 50 people. Iraqi teenager Ahmed Hassan, 18, was found guilty of attempted murder
Colleagues and family members have paid tribute to the victims of the Westminster attack.
A colleague of PC Keith Palmer, the officer who was stabbed when he confronted Masood at the Palace of Westminster, said the officer was a "loyal friend", "always happy" and dedicated to his job, his daughter and his wife.
PC Shaun Cartwright said: "Keith loved being a police officer, he just wanted to help people and do his best.
"Most of all I will remember him as a family man who idolised his wife, daughter and his family; they're the important ones that I think about a year on from the Westminster attack."
Senior police officers are expected to attend a number of private memorial services later.
The sister of Andrea Cristea, a Romanian tourist who was hurled into the River Thames after Masood's car hit her, has spoken about how she refuses to dwell on her killer.
Speaking to BBC London, Magda Toi said thinking of Masood only made her angry. "My sister is dead and no-one and nothing will bring her back," she said.
Ms Toi told of how her family had been given hope when Ms Cristea was recovered from the river alive after surviving the 20ft (6m) fall from Westminster Bridge.
"We desperately hoped that she wouldn't die. She survived the brain operation and we thought – she has a chance. But she didn't."
Ms Cristea died two weeks later and was buried in her hometown of Constanța, Romania.
Mr Khan said: "Londoners will never forget the horrific terror attacks on our city in 2017.
"We will never forget the bravery of our emergency services and first responders who ran towards danger while urging the rest of us to run to safety."