A auto careered into pedestrians on Westminster Bridge before the man, armed with a knife, tried to enter the Parliamentary estate, where he and PC Palmer died.
Police constable Keith Palmer, a 15-year veteran, was killed at Parliament Wednesday afternoon.
"One of those who died today was a police officer - PC Keith Palmer - a member of our Parliamentary and Diplomatic Protection Command".
Three pedestrians were mowed down on Westminster Bridge when the attacker mounted a pavement in a Hyundai 4x4.
Labour MP Barry Gardiner was also caught up in the lockdown and posted a picture of armed police moving through parliament.
The assailant was shot dead near London's Houses of Parliament after the chaotic rampage.
"Heartbroken" former colleague Conservative MP James Cleverly paid tribute to the "lovely man" he had known for a quarter of a century. We served together in the Royal Artillery before he became a copper.
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A total of five people - including the perpetrator - died and around 40 others were injured in the horrific attack.
Britain's allies reacted with shock and vowed to stand with London in the fight against terror, AFP says.
Social media users shared an altered image of a London Underground sign reading "We Are Not Afraid" and the hashtag #PrayforLondon trended on Twitter.
There are reports he was not armed at the time the lone wolf attacker lunged at him with two knives; instantly several of his colleagues rushed to his aid but he died of his wounds.
Three French pupils on a school trip were among those hurt and a seriously injured woman was rescued from the River Thames following the incident.
At least 20 people were wounded and some were said to have sustained "catastrophic" injuries.