An American tourist has shared a photo of herself with Pc Keith Palmer outside Parliament, less than an hour before the officer's death in the Westminster terror attack.

Staci Martin, visiting from Florida, said she wanted to share the image for "people to know what a nice person he was and how fragile life can be".

Around 45 minutes later, Khalid Masood, formerly known as Adrian Elms, drove a car into a number of pedestrians on Westminster Bridge, before stabbing and killing Pc Palmer, a 48-year-old father-of-two, at the gates to the Palace of Westminster.

Of the photo, Ms Martin told ABC News: "It's my first time in London and I see his hat and I'm like 'I have to take a picture of him with his hat'."

"I walked up to him and said 'do you mind if I take a picture?' He said 'no problem'. He was really nice."

She said she wanted to ensure the photo got back to his family, believing it must be one of the last photos taken of him alive.

Another tourist who met Pc Palmer at the same Palace gate last year paid tribute to the slain officer, saying his murder "sent a shiver down my spine".

Sharing a photo of himself and his two daughters with the constable, Andrew Thorogood, a jeweller from Alice Springs, Northern Territory in Australia, spoke of a "genuinely nice bloke" whom he had invited to visit.

"We were in London last October and visited the Houses of Parliament. An officer by the name of Keith Palmer was happy to pose with us for a photo once he learned that we had travelled all the way from Alice Springs in the Northern Territory, Australia," Mr Thorogood, 41, wrote.

"I spoke with Keith for quite a while and found him to be a genuinely nice bloke. He said he would love to visit Australia with his family one day.

"The girls suggested if he did make it to Australia, he should visit Alice Springs and we would show him how special a place it is.

"That will never happen now, all because he was doing his job and trying to keep people safe in the face of yet another crazy terror attack. Our thoughts are with his wife and family.

"With everyone so suspicious of everyone else these days, he was happy to chat and smile for a photo with a bunch of Territorians on holiday whilst still remaining vigilant and carrying out his duties as a police officer."

Mr Thorogood later told the Press Association: "The last 24 hours have been somewhat surreal. I can't begin to imagine what his wife is feeling.

"It strikes a chord because my wife is from Essex and we have a close affinity with the UK.

"The brief conversation and photo we had with Keith just makes it more personal."

Three other innocent victims - Kurt Cochran, Aysha Frade and Leslie Rhodes - have died following the attack, while many more were injured.