Mike O'Brien was three days shy of his twentieth birthday in 1988 when he was arrested for a murder he did not commit. His son was just three, his young wife was eight months pregnant for a second time, and he had forged a new relationship with his alcoholic father after a lifetime of acrimony.

Two years ago O'Brien finally won his freedom. He is now 33 and has only ashes to show for his life: his second child, a daughter, died in her cot when she was two months old; his wife left him a couple of months later for a string of other men; his father, broken by his son's imprisonment, drank himself into the grave.

'When I fought for freedom during all those years in prison, I thought I was fighting to get out from behind bars,' said O'Brien. 'But prison smashed my life to bits. It was when I got out that my troubles really started.'

O'Brien was one of the Cardiff Newsagent Three, wrongfully jailed for the 1987 murder of newsagent Philip Saunders, beaten to death with a shovel after a botched robbery near his home.

To this day, O'Brien cannot understand why the police decided to sacrifice his life to close the Saunders case, but when he left prison he swore to become the Crown Prosecution Service's worst nightmare, destroying the system which destroyed him with a strong and careful hand.

He fulfilled his promise last week with the launch of the Miscarriages of Justice Organisation, Mojo, alongside Paddy Hill, one of the Birmingham Six, in a Pugin-wallpapered parliamentary hall with the support of the Prime Minister and the Chancellor. After the ceremony, O'Brien told the full story of his nightmare for the first time, emphasising that his tale, however awful, is just one of dozens of similar stories throughout the country.

O'Brien was born Michael Bonello in Ely, Cardiff, in 1977, the second of six children brought up by his mother Marlene and their alcoholic and abusive step-father, Jimmy O'Brien. He was a lonely, painfully shy boy who escaped the constant bullying at school by wandering around the local wasteland until it was late enough to go home each night.

At 15, O'Brien left school and found work as a shelf stacker. Two years later he met his future wife: Donna Purcigo was just 15 but they fell in love immediately. When they married in 1986, Donna was eight months pregnant and they were both delighted. 'It was the most secure I'd felt in a very long time,' said O'Brien.

Kyle, their son, was born on 24 August, 1986. 'I couldn't wait to get him home and do all the silly things dads do,' said O'Brien. 'I wanted to bath him and play with him. I'd never been so happy.'

Despite their happiness, the marriage started disintegrating. Donna began having affairs and O'Brien followed suit. But the collapse of his marriage disturbed him and when he was attacked and sexually abused by an older man soon afterwards he fell into a deep depression, spending time in a mental hospital.

Donna became pregnant again and O'Brien turned to drink: 'I'd always refused to touch alcohol because I'd seen what it had done to Jimmy, but I didn't even think about Jimmy; I just turned to the first thing that gave me comfort.'

On 12 October, 1987 - the night Saunders was murdered - O'Brien, his brother-in-law Ellis Sherwood, and Darren Hall stole a car and went joyriding. 'It was the first time I'd done anything worse than nicking crisps, but I went along because I was too cowardly to say no,' he admitted. O'Brien was still in bed next morning when the police called. There were three cars and 10 officers. They surrounded the house and smashed the door down with a sledgehammer.

At the police station, O'Brien was refused a solicitor. 'They were shouting and screaming at me. They lied to get me confused and twisted my words to try to get me to admit to things I hadn't said. They wanted Ellis more than they wanted me because he already had a criminal record, but they said that if I didn't give evidence against him they'd take me down too. And that's what they did.'

Unable to keep O'Brien in custody for more than 72 hours without charging him, the police freed him with a promise that it was only just beginning. One week later, they burst into O'Brien's house a second time and took the 19-year-old to the station. This time he was charged with murder and taken to Cardiff prison on remand.

When Kylie, O'Brien's daughter, was born a few weeks later he was not allowed to attend the birth and could only see her for 15 minutes each day. 'The closing-up I did of my emotions in those first months was extreme, but I had no choice,' he said. 'I could only survive what was happening by cutting off entirely.'

Six months later, O'Brien had a visit from his solicitor. 'He said he had something important to tell me.' A priest was waiting for him and O'Brien was told to sit down: his daughter had died of cot death that morning. 'It wouldn't sink in,' he said. 'Half of me died that day; it was burnt out at the root.'

Two months later, O'Brien had another visit: Donna was leaving him. 'She smashed me to bits with that news,' he said. 'The rest of me died a few weeks later at the trial.'

The trial lasted for three weeks. 'As soon as it began, I knew we were finished,' said O'Brien. 'I spent most of the time just staring at my family: I wanted to make sure I'd remember them.'

O'Brien was sent to Long Lartin prison. 'I saw seven men murdered in front of me during my time there; one was stabbed in his groin, heart and his lungs,' he said. 'I had to learn to defend myself: I carried a knife in one pocket and a screwdriver in the other.'

O'Brien met Carl Bridgewater, Vincent Hickey and Michael Shirley during his years in prison [This must be a printer's error - Vincent Hickey was convicted of the murder of Carl Bridgewater! Ed.], but the support of other innocent men was lost on O'Brien: he was losing control, taking all the drugs he could find and retreating into his own world.

Then he met Paddy Hill. 'Paddy was a hero to us all. He took me in hand. He told me that unless I started using my anger constructively, I'd spend the rest of my life behind bars. Something flickered inside me then for the first time in years. I lay awake that night thinking, and by the morning I knew that he was right,' said O'Brien.

O'Brien visited the prison psychologist and the doctor. He began weaning himself off drugs and studying law. 'I saw how wrongly I'd been treated and how many laws the police had broken,' he said. 'My mind began opening up and I began fighting back.' O'Brien divided up his day: three hours for criminal law, one hour for human rights and one hour for prison law.

He organised press releases around every anniversary possible: the date of the murder and the date of his trial. He co-ordinated the first mass hunger strike across 20 prisons around the country.

'We began getting headlines and I realised the scope for publicity was there.' He successfully sued the then Home Secretary, Michael Howard, twice: once for allowing prisoners to be stripped and searched on CCTV, and again when Howard banned journalists from talking to prisoners, winning his case in 1996. When Jack Straw reneged on the ruling a year later, O'Brien started the fight again, winning again in 1999.

'The screws tried to intimidate and threaten me. They moved me away from my family and took away privileges, but I tape-recorded their threats and each time I won,' he said. With Michael Mansfield QC as his barrister, O'Brien's efforts were rewarded: on 18 December, 1999, the Court of Appeal took under 20 minutes to quash the convictions of all three men. But after fighting for his freedom for 11 years, O'Brien found that problems began anew.

'For years I'd been living on my anger and frustration,' he said. 'That doesn't go away now I'm out: I never got a chance to see my son growing up, my daughter and father are dead, my wife is gone and my health, both mentally and physically, is wrecked.

'I thought I'd find freedom outside the prison, but it's not here,' he added. 'I don't know if I'll ever find it now.'

Five cases taken up by Mojo

• Headmaster Sion Jenkins was jailed for life in 1997 after being found guilty of murdering his foster daughter Billie-Jo, 13, at their home in Sussex. Jenkins claimed he found her body after returning from shopping but he was found guilty after forensic experts discovered blood spots on his clothes.

• Satpal Ram was convicted of murder after killing Clarke Pearce at a Birmingham curry house in 1986, despite witnesses' claims that he was protecting himself against a racist attack.

• Mark Barnsley, a 33-year-old father of three from Sheffield, was sentenced to 12 years for grievous bodily harm after being attacked by 15 students in a pub in 1994. He claims the students were drunk and under the influence of drugs.

• Eddie Gilfoyle was sentenced to life in 1993 for killing his pregnant wife and faking her suicide. His counsel, Michael Mansfield QC, maintained that crucial evidence had been destroyed or discarded.

• Jamil Chowdhary was 25 when he was jailed for life in 1992 for the murder of a teenager during a robbery at a petrol station in Winchester. Chowdhary claims he was framed.