The Met said Pc Fletcher's murder investigation remains open
The family and colleagues of murdered police constable Yvonne Fletcher have marked the 25th anniversary of her death in a central London service.
Pc Fletcher, 25, was shot while policing a protest outside the Libyan embassy in London on 17 April 1984.
No arrests have been made in connection with her killing, which sparked an 11-day siege at the embassy.
Met Police Commissioner Sir Paul Stephenson said: "We will [not cease trying] to identify those responsible."
Pc Fletcher's parents, Tim and Queenie, Sir Paul, Assistant Commissioner John Yates and Police Memorial Trust founder Michael Winner laid flowers at the officer's memorial before attending a service at St James's Church in Piccadilly.
'Thin blue line'
Pc Fletcher, originally from Semley, near Shaftesbury, was gunned down as she policed a demonstration against the Libyan President Muammar Gaddafi's regime outside the the Libyan People's Bureau in St James's Square.
Investigators believe the bullet which killed her was fired from inside the embassy by a sniper from a first-floor window.
The siege that followed led to 20 embassy staff being allowed to leave the UK under diplomatic immunity laws.
Yvonne Fletcher is thought to have been shot by a sniper
Scotland Yard said the murder investigation "remains open" and believes the killer could have been smuggled out of the country along with embassy staff.
Pc Fletcher's death sparked the breakdown of diplomatic relations between the two countries, which was restored only in 1999.
A joint investigation between British and Libyan police into the murder was launched in 2004.
Since then, Scotland Yard detectives have visited the country five times, the latest in 2007 to collect evidence and statements.
Sir Paul said: "Yvonne's murder is a stark reminder of the huge risk police officers can face on a daily basis while doing their duty.
"This tragic and unprovoked attack on someone who was working to ensure public safety remains an horrific event in policing history."
Mr Winner added: "Take away the thin blue line that protects us from evil and anarchy and chaos would follow. We have a lot to thank them for."