Metropolitan Police commissioner Bernard Hogan-Howe appealed for information today about the murder of a "courageous" officer on the 20th anniversary of his death.
The police chief spoke at a memorial service for Detective Constable Jim Morrison, who was killed outside the Indian High Commission in central London on December 13 1991.
The 26-year-old was off-duty waiting for his wife Victoria in Covent Garden when he saw a handbag thief and chased him.
When he cornered the suspect, he was stabbed, and the thief was seen running off into the Strand.
The case has never been solved.
Today members of his family, his friends and former colleagues gathered at the spot where he was killed to remember the officer, who was based at Bow Street police station.
Mr Hogan-Howe, who paid tribute to Det Con Morrison during the service, said afterwards that police were launching a fresh appeal to find his killer.
He 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."
He added: "Day in, day out, the police officers at the Met do some incredibly brave things.
"Only very rarely do police officers lose their lives."
The Rev Philip Majcher, who led the half-hour service, said Det Con Morrison's "courage and dedication is an inspiration" to others.
He said the officer "died courageously in the course of duty as a Metropolitan Police officer protecting the weak and vulnerable from those who would prey upon them".
Det Con Morrison made more arrests in the area than any other serving police officer at the time, according to his colleagues.
The renewed police appeal comes with a £20,000 reward for information leading to the arrest and prosecution of the person responsible for his death.
Det Con Morrison's widow Victoria, who attended the service, said in a statement: "Jim's death, for me and the family, was completely devastating.
"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."
Detective Chief Inspector Amanda Hargreaves, from the Met Police's homicide and serious crime command, also appealed for the public's help and said the North African and Algerian community could play an important part in solving the crime.
She said: "Det Con Morrison was a talented young officer with a bright and promising future, which was taken away from him when he was brutally attacked.
"Over the years, officers have remained diligent in attempting to solve this case. Despite the passage of time, I believe there are people who still hold vital information about who is responsible for Det Con Morrison's murder.
"Allegiances and loyalties can change in the intervening years and I appeal directly to those individuals with information to get in contact.
"I ask people to think back to December 1991. Were you in the Covent Garden/Aldwych area on the night of Det Con Morrison's murder? Do you have information that could help solve this crime?"
The suspect was described as being 5ft 10in and aged 27 to 30 at the time.
He is believed to be of North African or Algerian origin, clean-shaven, with dark collar-length hair with distinctive tight curls at the front.
He was wearing a waist-length tan or brown leather jacket.
To coincide with the service in the capital, a graveside memorial service was also being held in the Isle of Harris, Outer Hebrides, where Det Con Morrison was buried, led by his best friend and serving police sergeant Colin Geddes.
During the service in London flower tributes were placed at the memorial stone which marks where Det Con Morrison fell.
It was laid in 1994 by the Michael Winner Trust, which recognises officers murdered in the course of duty.
The trust has unveiled more than 30 such memorials to officers around the UK.
Among the most memorable deaths are those of WPc Yvonne Fletcher, who was shot dead while policing a protest outside the Libyan embassy in London in 1984 and that of Pc Keith Blakelock, who was killed by a machete-wielding gang during the Broadwater Farm riots in Tottenham, north London, a year later.