A Libyan man has been arrested on suspicion of conspiring to murder Pc Yvonne Fletcher more than 31 years after she was shot dead.
The suspect - in his 50s - was detained by counter-terrorism officers in south east England in what police described as a "significant turning point" in the inquiry into the death of the officer, who was gunned down aged 25 during a protest outside the Libyan embassy in London in April 1984.
In addition to conspiracy to murder, the man was arrested on suspicion of money laundering along with a woman in her 40s and a man in his 30s. They are also Libyans and were held in south east England and London respectively. Searches are being carried out at several locations around the country.
Announcing the development, detectives said the focus of the investigation has now shifted to an alleged conspiracy to murder the officer and demonstrators, disclosed they believe two guns were used, and launched a global appeal for new information.
Commander Richard Walton, the head of the Met's counter terrorism command, said: "Over the past 31 years we have never lost our resolve to solve this case, to bring to justice those who conspired to commit this act of murder.
"It remains one of the saddest and darkest days in the history of British policing."
Following the announcement, the officer's family made an emotional appeal for information, saying her father Tim died recently with his "one regret" that he had "never witnessed any justice".
Pc Fletcher was killed as she policed a demonstration against Colonel Muammar Gaddafi outside the Libyan People's Bureau in St James's Square.
The shooting sparked a 10-day siege of the building before 30 of the occupants were deported back to Libya.
Despite repeated efforts to secure a breakthrough, no-one has ever been prosecuted over Pc Fletcher's killing.
Police said the demise of Gaddafi, who died in 2011 during civil war in Libya, and the regime change that followed had "provided access to new lines of inquiry and displaced many people outside of Libya". British detectives have visited Libya on seven occasions since 2012.
Police refused to provide specific details of the arrests or how the money laundering and murder inquiries may be linked.
However, Mr Walton said: "We do believe today's arrests mark a significant turning point in our long-standing investigation."
At around 10am on the morning of April 17, 1984, a large group of anti-Gaddafi campaigners gathered outside the embassy.
Meanwhile, a group of supporters of the dictator held a counter demonstration nearby, which investigators suspect was organised and co-ordinated by individuals within the bureau.
The Met believe the incident was part of the so-called "stray dogs campaign" orchestrated from Libya to attack overseas dissidents and their interests at the time.
At about 10.17am, a number of shots were fired from within the building, hitting the officer who suffered a fatal shot to the back.
Mr Walton said: "I can tell you today the we believe two firearms were used - a pistol and an automatic weapon."
Ten men who were protesting against Gaddafi were also shot and injured.
Scotland Yard has now released 14 images of individuals from the pro-Gaddafi group who they want to speak to, as well as video footage of the demonstration and shooting.
The 54-second clip shows the chaotic moments after the shots were fired.
Dozens of demonstrators fall to the ground before Pc Fletcher is seen stricken on the ground surrounded by colleagues.
A reward of up to £50,000 is being offered for information leading to the identification, arrest and prosecution of those responsible for the murder.
Police also launched a massive social media campaign to appeal for those present on the day or others who have information to come forward.
"Allegiances change and we hope with the passage of time, witnesses who have not spoken to us will examine their consciences and come forward," said Mr Walton.