Relatives and friends of people who died while in police custody have marched to Downing Street to demand justice for their loved ones.
The United Families and Friends group, which organised the event, said about 300 protesters took part in the annual procession, which started in Trafalgar Square and marched down Whitehall.
Activists carried placards which read "No Justice No Peace" and banners in memory of those such as musician Sean Rigg, who died after being restrained at Brixton police station, south London, in August 2008.
Deborah Coles, director for the Inquest campaign group, said the procession was an "important but poignant" day for the families.
She added: "Many families feel betrayed by a system that has let them down.The same issues repeat themselves time and again despite the empty platitudes from Government ministers that lessons will be learned."
The son of murdered police officer Pc Geoffrey Fox has told ITV News he would be "happy" to execute killer Harry Roberts himself.
Speaking after Roberts' release was approved by the Parole Board, Paul Fox said: "If they gave me the opportunity I'd be quite happy to execute him myself.
"I think [if] you live by the sword you die by the sword... personally I think the whole justice system is a joke."
The widow of murdered police officer David Stanley Bertram Wombwell has told ITV News she resents the fact killer Harry Roberts is to be released.
Gillian Wombwell said:
I was informed all the way that there was a possibility Roberts would be released.
I have voiced my reservations again and again. We resent the fact that he will be released. Our sentence is for life and so should his be.
The chief of the Metropolitan Police, Sir Bernard Hogan-Howe, has responded to the expected release of police killer Harry Roberts:
Met Police Chief Sir Bernard Hogan-Howe on realise of 3xPolice murderer Harry Roberts: 'in this case "life" should have meant what it said'
The daughter of a murdered police officer has said she is "sickened" to learn that her father's killer is to be released without any prior warning.
Harry Roberts, now 78, opened fire on three unarmed police officers in 1966. He has served 45 years of a life sentence.
Mandy Fox, the youngest daughter of Pc Geoffrey Fox, told ITV News:
With reference to the release of Harry Roberts by the parole board, I would like to complain strongly to the ministry of justice for not informing our family that this release had been granted and was imminent. It has impacted severely on myself as I have health problems compounded greatly by the devastation caused by Mr Roberts and company and it sickens me to know that this is happening without any prior notice being given in order to prepare myself mentally for what is a very emotional time.
What signal does this show our courageous serving police officers throughout the country, who put their lives on the line daily for our protection and safety, it is an utter disgrace and should never have been aloud [sic].
We have not further comments and wish not to be disturbed at this time.
A man who was at the scene of the murder of three unarmed policemen has told ITV News it is a "disgrace" that one of the men responsible for their murders is being released from prison.
Glen Hazell, who was 11, was playing with friends in Shepherd's Bush when Harry Roberts and two associates opened fire on the officers in 1966.
The Parole Board today announced that Roberts, who was handed a life sentence, will be released after serving 48 years, a decision that has sparked fury among police groups.
"I don't think he should be allowed anywhere near the public...it's a disgrace," he told ITV News Senior Correspondent Emma Murphy.
People will be "absolutely sickened" by the release of convicted police killer Harry Roberts, Boris Johnson said.
The Mayor London said "life should mean life" as he criticised the decision to free Roberts, who has handed a life sentence for murdering three police officers in 1966.
"It is perfectly possible that the Parole Board has seen new evidence, I don't know. What I do know is that Londoners will be absolutely sickened by this news," Mr Johnson said.
"They will find it hard to understand how a man who shot dead three police officers in this city in the most horrific fashion can now enjoy the freedom he denied his victims.
"To my mind, in the case of the murder of a police officer, life should mean life."
Home Secretary Theresa May has declined to comment directly on the decision to release convicted police killer Harry Roberts, saying it was a "decision by the independent Parole Board".
The Parole Board has confirmed that a panel has directed the release of Harry Roberts, but declined to reveal when the convicted police killer would leave prison.
Police groups have reacted furiously to the decision.
We can confirm that a three-member panel of the board has directed the release of Harry Roberts.
The decision to release is a matter for the board, which is independent - arrangements and the date of the release are a matter for the Secretary of State for Justice.
We are unable to comment further on the details of this case.
The chair of the Metropolitan Police Federation said the imminent release of convicted killer Harry Roberts is a "total betrayal of policing by the criminal justice system".
Roberts, who was jailed for life for the murder of three unarmed policemen in 1966, could leave prison within days following the Parole Board's decision.
Reacting to news of Roberts' impending release, John Tully tweeted: "A total betrayal of policing by the criminal justice system.
"This man should never see the light of day again, life should mean life."