A Libyan man arrested over an alleged conspiracy to murder Pc Yvonne Fletcher more than 30 years after she was shot dead has been bailed by police.

The suspect, who is in his 50s, was arrested by counter-terrorism officers in south-east England in relation to the fatal shooting outside the Libyan embassy in London in April 1984.

The 25-year-old officer was gunned down during a protest in what police have described as "one of the saddest and darkest days in the history of British policing".

In addition to conspiracy to murder, the man was arrested on suspicion of money laundering, but has been bailed to a later date.

He was arrested along with two other Libyan nationals, a woman in her 40s and a man in his 30s, who were earlier bailed until late January, while a search at an address in south-east England continues.

Detectives said they have "never lost the resolve to bring to justice those who conspired to commit this act of murder" and the investigation is focusing on a conspiracy to the shooting.

Commander Richard Walton, the head of the Met's counter-terrorism command, said: "The investigation is now focusing on new lines of inquiry relating to a conspiracy to murder prior to the shooting of Yvonne, and we are acting upon fresh new evidence."

The officer's family have 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 on April 17 1984.

The shooting sparked a 10-day siege of the building before 30 of the occupants were deported back to Libya. No-one has ever been prosecuted over the killing.

A reward of up to £50,000 is now being offered for information leading to the identification, arrest and prosecution of those responsible. Police said the demise of Gaddafi - who was killed in 2011 during civil war in Libya - and the regime change that followed had "provided access to new lines of inquiry". Officers say they are acting on fresh evidence.

British detectives have visited Libya on seven occasions since 2012. Scotland Yard refused to provide specific details of the arrests or how the alleged money laundering and murder inquiries may be linked.

Investigators 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 on the day, a number of shots were fired from within the building, and the unarmed officer suffered a fatal shot to the back. Ten anti-Gaddafi protesters were also injured.

Scotland Yard released video footage showing 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.

Police appealed 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.

Images of 14 pro-Gaddafi demonstrators who were also gathered outside the building on the day and who police wish to trace have been released.

Mr Walton said: "There were a number of witnesses who were part of the pro-Gaddafi counter-demonstration whom we have never spoken to.

"We appeal particularly to these people to come forward and speak to us now, even after 31 years."