"Heartbroken" former colleague, Conservative MP James Cleverly, paid tribute to the "lovely man" he had known for a quarter of a century.
He wrote of the 48-year-old father and husband: "I've known Keith for 25 years. We served together in the Royal Artillery before he became a copper.
"A lovely man, a friend. I'm heartbroken.
"My thoughts are with the family, friends and colleagues of Pc Keith Palmer. A brave man."
Scotland Yard's top anti-terror officer Mark Rowley said of his colleague: "He was someone who left for work today expecting to return home at the end of his shift - and he had every right to expect that would happen."
Greater Manchester Police chief constable Ian Hopkins added: "#RIP PC Keith Palmer, brutally murdered today carrying out his duty helping to protect Parliament and the freedom of our Country."
The Metropolitan Police Federation, which represents officers from the force, said: "PC Keith Palmer. Our brave Met #Police colleague who today made the ultimate sacrifice #LestWeForget"
A chorus of thanks also came from politicians on all sides of the House.
Home Affairs Select Committee chairwoman Yvette Cooper, who was in the Palace of Westminster when the terrorist burst through the gates, said: "Thank you Pc Keith Palmer - to your bravery many others owe their lives and safety"
Labour MP Barry Gardiner was also caught up in the lockdown and posted a picture of armed police moving through Parliament.
He tweeted: "Going to bed alive, safe and well thanks to Pc Keith Palmer. God bless him, his family and all those killed or injured. Terror will not win."
Geoffrey Cox, Conservative MP, wrote: "Deeply sad that a brave Pc, Keith Palmer, died in the service of his country, doing his duty today. We should be proud to know such men."
MP Chi Onwurah tweeted: "As I left Parliament a police officer apologised for keeping us waiting. Cannot overstate courage & grace of those who protect us Thank you."
One woman who said she had her life saved by Pc Palmer when she worked as a police officer paid tribute to the "absolutely lovely man".
Giving her name only as Nina, from Orpington, south-east London, she told radio station LBC: "He actually did save my life in a car accident on duty, I was a police officer.
"I was a police officer at the time, in 2007.
"I had a car accident on duty, I was the passenger. I was the passenger and he was one of the police officers who actually saved my life.
"I owe him everything. He was a wonderful dad, and wonderful husband to his wife and everything."
Reflecting on the officer's attitude to his work, she said: "He didn't just do his job to go and get paid. He actually cared about the people.
"He really did, he was a lovely man, absolutely lovely man, and everyone's going to really miss him."