Metropolitan Police commissioner Bernard Hogan-Howe has appealed for information to catch the killer of a "courageous" officer 20 years ago.
Det Con Jim Morrison, 26, was off duty when he chased a handbag thief and was stabbed outside the Indian High Commission in Aldwych, central London.
The crime, on 13 December 1991, has never been solved.
Mr Morrison's widow Victoria and Mr Bernard Hogan-Howe attended a service at the spot where the officer died.
A £20,000 reward has been offered to catch Mr Morrison's killer.
The service to remember him took place in Montreal Place, close to India Place where a memorial stone was laid by the Michael Winner Trust in 1994 to mark the spot where the officer fell.
Mr Hogan-Howe said: "Jim was incredibly courageous. He took on a person who'd snatched a bag, and he gave his life.
"Here we are, 20 years after, and we still haven't caught that person. There will be somebody out there who knows who it is.
"It's vital that we mark his memory by trying to find his killer, and anybody out there who knows who did it, please tell us."
While officers do "incredibly brave things" everyday, it was "only very rarely" they lose their lives, sad Mr Hogan-Howe.
Mr Morrison was waiting for his 24-year-old wife in Covent Garden when he saw the suspected thief and chased him.
He was stabbed when he cornered the thief, who was later seen running away towards the Strand.
The suspect was described as a 5ft 10in (1.7m) tall man, who was aged between 27 and 30 and of North African or Algerian origin.
Mrs Morrison, who married the officer in 1988, said: "I was 24 years old and in the first few years of marriage I was suddenly widowed and my whole life was turned upside down.
"It has been very traumatic for me and for Jim's family. Even 20 years on, it is still very, very hard.
"I appeal to anyone who has information that could help solve Jim's murder to please come forward and help us to achieve justice for Jim's family and friends."
The Rev Philip Majcher, who led the half-hour service, said Mr Morrison had "died courageously in the course of duty as a Metropolitan Police officer protecting the weak and vulnerable from those who would prey upon them".
Launching the fresh appeal, Det Ch Insp Amanda Hargreaves said Mr Morrison had been "a talented young officer with a bright and promising future" and called on those with information to come forward.
"In particular, it is believed the North African and Algerian community could play an important part in solving this crime," she said.