- Video report by ITV News Correspondent Angus Walker
MPs and police are among those who have solemnly marked the one year anniversary of the Westminster Bridge attack.
Ahead of business in the House of Commons, Speaker John Bercow asked those in the chamber to pause "in respectful memory" of those who lost their lives in the attack on March 22 last year.
A service was later held at Westminster Hall, and Archbishop of Canterbury, Justin Welby, told the gathering: "A year ago, darkness struck across Westminster Bridge and in this palace.
"It spread across the bridge like a snake, driving to left and right, killing and harming."
Reverend Rose Hudson-Wilkin, the Speaker's chaplain, told the service: "A year ago today on this estate and on Westminster Bridge we were visited by what I regard as evil."
Elsewhere, senior police officers were expected to attend a number of private memorial services, although details of those have not been made public.
And this evening, the message "London United" will be projected onto the Houses of Parliament, London Bridge, Finsbury Park Mosque and Parsons Green Tube station tonight – all places hit by terrorist attacks last year.
In the Westminster attack, five people were killed and another 50 injured when attacker Khalid Masood drove a 4x4 into a crowd on Westminster Bridge then launched a knife attack, before he was shot and killed outside parliament.
Among those murdered was Keith Palmer, an unarmed constable who was on duty at parliament that day and was stabbed by Masood.
In a statement released through Scotland Yard, friend and former colleague PC Shaun Cartwright said: "Keith loved being a police officer, he just wanted to help people and do his best.
"Keith was always happy, always the first to help anyone out, first in to work and the last one out.
"He was a proud and courageous police officer who did his job and never wanted any fuss or to be the centre of attention. Keith was a true and loyal friend, utterly reliable.
"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."
Another person who distinguished themselves that day was MP Tobias Ellwood, who attempted to save the life of the injured PC Palmer.
- MP Tobias Ellwood recalls day of attack
Speaking to ITV News, Mr Ellwood, who received medical training in the services, recalled his memories of the tragic day.
"I saw PC Keith Palmer on the ground and he was bleeding profusely," he said.
"Some officers were trying to help him. Being medically trained I thought I could offer some help.
"I arrived and there was a pulse and there was profuse bleeding and the medics came and I thought I’d be pushed to the side but they asked me to keep doing what I was doing," he added.
After the helicopter landed Mr Ellwood said he thought the victim would be taken away, but the casualty needed stabilising and he was asked to carry on assisting.
"It was very sad to be told that it wouldn't be possible to save his life and that too much blood had been lost," he said.
A new UK Police Memorial being built in Staffordshire to commemorate officers killed on duty will include PC Palmer's name alongside 1,400 others.
Another 4,000 of those who died on duty will be remembered inside the memorial.
Supporters have been asked to donate £5 towards the scheme, for which £3.1 million has already been raised towards a total cost of £4 million.
It includes an education programme to teach children about policing and support scheme for bereaved families.
Sir Hugh Orde, chair of trustees of the UK Police Memorial, said: "This has been an unprecedented year for policing with officers and staff repeatedly putting themselves in danger to keep the public safe and protecting them from harm.
"Sadly, in March last year we saw one of my policing colleagues PC Keith Palmer lose his life during a terrorist attack in London whilst protecting our freedoms and safeguarding our democracy," he said.
"Keith's death put greater emphasis on the need to create a memorial where the nation can go to commemorate our police service."