A man has described how he fought off the London Bridge knifeman with a narwhal tusk before pinning him to the ground to help end the attack.
Darryn Frost, 38, was seen in pictures confronting Usman Khan, 28, who was armed with two knives.
The civil servant has spoken of his "deep hurt" at not being able to save Jack Merritt and Saskia Jones who were killed by Khan on 29 November.
Khan, who was wearing a fake suicide vest, was shot dead by police.
Mr Frost, who works in communications at the Ministry of Justice, told how he grabbed the narwhal tusk from the wall in Fishmongers' Hall as Khan launched his attack during a prisoner rehabilitation event near London Bridge.
The South African-born Londoner then chased Khan onto the bridge, where footage captured him and fellow members of the public fending off the attacker.
Mr Frost, whose identity was unknown until now, can be seen pinning Khan to the ground before being pulled away by a police officer. The terrorist was shot dead moments later.
Khan was out on licence from prison when he killed Mr Merritt and Ms Jones and injured three others in the stabbing attack.
Speaking to the PA news agency, Mr Frost said he was attending the rehabilitation event with colleagues when he heard a commotion downstairs.
He said he then grabbed the decorative whale tusk, which had been hanging on a wall: "A few of us rushed to the scene. I took a narwhal tusk from the wall and used it to defend myself and others from the attacker.
"Another man was holding the attacker at bay with a wooden chair. I ran down the stairs, stood next to the man with the chair, and the two of us confronted the attacker."
'Knives raised above his head'
Mr Frost added: "He had knives in both hands and, upon seeing me with the narwhal tusk, pointed at his midriff.
"He turned and spoke to me, then indicated he had an explosive device around his waist. At this point, the man next to me threw his chair at the attacker, who then started running towards him with knives raised above his head."
Mr Frost handed the tusk to the man next to him before heading back upstairs to find another one.
When he returned, he found the first tusk "shattered across the floor" and people fleeing the building.
He said: "Along with others, I pursued the attacker, tusk in hand, onto the bridge. We called out to warn the public of the danger and, after a struggle, managed to restrain him to the ground.
"At that point I was trying to isolate the blades by holding his wrists so that he could not hurt anyone or set off the device."
In the footage of the altercation, Mr Frost can be seen grappling with Khan on the ground before being pulled away by an officer - seconds before police shot the attacker.
Cambridge University graduates Ms Jones, 23, and Mr Merritt, 25, were stabbed to death in the attack.
They were also attending the conference organised by the university's programme called Learning Together.
Mr Frost said that after reading about the work of Ms Jones and Mr Merritt he is "convinced they represent all that is good in the world" and said he "will always feel the deep hurt of not being able to save them".
Three others - a man and two women - were also injured in the attack.
'Hope on that dark day'
Mr Frost, who has lived in the UK for 14 years, revealed how some of those hurt refused treatment until others more seriously wounded were helped. A kindness, he said, that "filled me with hope on that dark day".
He said he has given his account of the "terrible day" in an effort to urge people to unite against terrorism and raise money for the victims' families.
He said he was "eternally grateful" to everyone who came to help, and thanked the emergency services.
"Not only do I want to thank those who confronted the attacker, but also those who put themselves in danger to tend to the injured, relying on us to protect them while they cared for others."
Addressing the public, he said he hoped "the part I played in these terrible events can be used for good".
Among those who were first to tackle the knifeman was a porter known only as Lukasz. He was armed with another makeshift weapon - a pole - and was stabbed five times as he confronted Khan alongside Mr Frost.
In a statement released in the week after the attack, the Polish national said he had "acted instinctively" and called the attack "sad and pointless".
He joined Mr Frost and others in following Khan out of the building but his injuries forced him to stop at bollards at the end of the bridge.
Another man, John Crilly, 48, has told how he helped Mr Frost and Lukasz in the tussle.
Crilly, who was jailed for murder after a burglary went wrong, first fought Khan with a wooden lectern and then a fire extinguisher, all the while believing he was wearing a live suicide belt.
In the video footage, he is seen using the spray from the extinguisher to blind Khan, while Mr Frost held him back with the narwhal tusk.
Crilly said: "The spray distracted him if you watch the footage. And the guy with the tusk has been able to give him a prod which has unbalanced him."
Mr Frost said he had left out details of the attack out of respect for the victims, their families and the ongoing investigation and inquests.
He appealed for privacy to continue his recovery and urged the press to use its "amazing ability to connect with people" as a force for good to "unify this country".
"I feel we all have a duty to challenge the spread of fear, hatred or intolerance within our communities," he added.