Two war veterans were reportedly among the first to rush to the aide of PC Keith Palmer who was fatally stabbed in the Parliament terror attack.
Former Staff Sgt Tony Davis and Captain Mike Crofts reportedly administered specialist first aid techniques, which they learned in combat, to try to stem the bleeding from his wounds.
The men were leaving a Parliamentary meeting on boxing when they found themselves in the middle of the chaos.
Mr Davis said he could tell PC Palmer was in “a great deal of difficulty” and said it was his “instinct” to help him.
“I heard a fracas. I thought it was a demonstration. We were on the side of the barrier making our way to the exit. I saw some large chap brandishing two knives and attacking two policemen,” he said in an interview with ITV's This Morning.
“I leapt over the fence, instinct kicked in. The assailant was coming toward us, I saw one of the marksmen put three rounds in him.”
He described how initially he thought the police officer’s wounds were not serious, corroborated by reports from other witnesses who described Mr Palmer as moving and sitting up.
“Initially when he fell to the ground I put him in the recovery position. It didn’t look too bad although he was bleeding. He had wounds in his arm and on his ribcage,” he said.
Conservative MP Tobias Ellwood, a former Captain with the Royal Green Jackets, also attempted to save the policeman and reportedly administered mouth-to-mouth on the officer.
Police have arrested eight people in connection with the terror attack in which four, including the attacker, were killed and at least 40 injured.
They have named the suspected attacker as Khalid Masood, a man who is believed to have lived in Birmingham and was previously investigated by MI5.
Isis claimed responsibility for the Westminster attack saying a "soldier of the Islamic State" had carried out the atrocity.
The Metropolitan Police's top anti-terror officer confirmed police had searched six addresses and made seven arrests, and the Met later released a tweet to say they had made an eighth arrest.
Addressing the Commons, Prime Minister Theresa May said: "We are not afraid and our resolve will never waiver in the face of terrorism."