Lissie Harper, whose husband Pc Andrew Harper was killed on duty in August 2019 , said the death of a Metropolitan Police officer was “utterly devastating”.
She shared a Facebook post with the news, adding: “What is happening to our world.”
In a statement, she also condemned the "barbaric" attack, adding: “This is devastating news. No person should go to work never to return. No human being should be stripped of their life in a barbaric act of crime.
“Another hero has been taken from us in unwarranted violence. They protect us but who protects them? Another life is gone in a disgraceful act that reminds us of the danger our police officers face with every shift they begin.
“My heart is broken for yet another member of our blue line family, and all of his family, friends and colleagues who must now accept a life without him in it. My thoughts and love are resolutely with them.”
Mr Harper, a Thames Valley Police officer, died when he was caught in a tow rope and dragged along country lanes after trying to stop quad bike thieves in Berkshire in August 2019.
His three teenage killers were cleared of murder but convicted of manslaughter after an Old Bailey trial.
Ms Harper is campaigning for a change to the law which would see all those convicted of killing emergency workers receive a life sentence.
The Met Police officer was shot in a south London police station in the early hours of Friday morning and later died in hospital, police said.
Prime Minister Boris Johnson, London Mayor Sadiq Khan and Met Police commissioner Cressida Dick were among many politicians and police bosses who condemned the attack and offered their condolences to the family and friends of the officer.
My deepest condolences go to the family, friends and colleagues of the police officer who was killed in Croydon last night.
We owe a huge debt to those who risk their own lives to keep us safe.— Boris Johnson (@BorisJohnson)September 25, 2020
Mr Johnson said he offered his "deepest condolences" to the family, friends and colleagues of the officer who died. "We owe a huge debt to those who risk their own lives to keep us safe," he added.
Mr Khan said he was "devastated" by the killing and paid tribute to the "brave officer" who "paid the ultimate price for helping to keep Londoners safe."
“Tragic incidents like this are terrible reminders of the dangers our police officers face every single day,” he said.
He added: "My thoughts are also with the entire Metropolitan Police family, who I know will be deeply mourning their colleague at this extremely difficult time.
"I remain in close contact with the Commissioner to offer her and our Met officers and staff my support."
Devastated by this news. My heart goes out to the family of this brave officer, who has paid the ultimate price for helping to keep Londoners safe.
Tragic incidents like this are terrible reminders of the dangers our police officers face every single day. https://t.co/LQ7PHNxwJt— Sadiq Khan (@SadiqKhan)September 25, 2020
Met Police commissioner Ms Dick condemned the "tragic" attack and added her condolences, which she said had sent "shockwaves and sadness" through the force.
She said: “This is a truly shocking incident in which one of our colleagues has lost his life in the most tragic circumstances. My heart goes out to his family, direct colleagues and friends.
“We are currently supporting his family and also have a dedicated team providing support to the officers and those in the custody centre who witnessed the shooting.
“When a colleague dies in the line of duty the shockwaves and sadness reverberates throughout the Met and our communities. Policing is a family, within London and nationally, and we will all deeply mourn our colleague.
The investigation into the officer's death is ongoing, she said.
The man's fellow officers also shared their sadness at his death. Stuart Hutson said he was one of the officers that responded first to the incident.
He added in a tweet: "This morning my team and I responded to the worst possible radio transmission from custody, words and scenes I shall never forget.
"The unimaginable happened to our police family. We have lost not only a good skipper but also a real gentleman. One of the best. RIP brother".