The 24-year-old Briton charged with the massacre of 13 people and then imprisoned for four months in the Central African Republic has told the full story of his remarkable release following a prison riot.

"The good news is I'm out," David Simpson, who faced a possible death penalty, told the Observer. "The bad news, or at least the strange news, is how it happened."

Simpson's story has always strained at the bounds of credibility – at one point he was accused of using witchcraft and magic potions to lull his victims to sleep. Last week it took a turn from the merely bizarre to the truly outlandish. After facing charges of ritualistic murder, in all likelihood carried out by the warlord Joseph Kony's Lord's Resistance Army, last Thursday he became caught up in the country's biggest ever jailbreak.

Simpson had been working in the vast wilderness in the east of the country, managing a 20,000 sq km concession for a safari company, when he stumbled across decomposing bodies in March. A young man who left his factory job in Sheffield to go in search of adventure, he had reported the deaths to the authorities, but a week later found himself arrested then charged with carrying out the crime himself. He was imprisoned in the country's capital, Bangui, along with 12 other staff of the safari company.

On Thursday afternoon, "just after we'd finished lunch", anti-government protesters stormed the jail, surrounded by high walls and razor wires and guarded by men with Kalashnikovs.

It was mayhem, Simpson said. The prison courtyard was full of looters. "I raced downstairs to try to padlock the stairs to our cell, but it was too late; they were already inside. I ran back up to the cell and we tried to barricade ourselves in. We held the door with our bodies for about five or six minutes then they burst through. There was just a wall of men with machetes looking at us. They couldn't believe they'd found a white man. They looked as shocked as I was."

Simpson was thrown to the ground, punched and kicked. "Everything was stolen. They took the shoes off my feet. They took the shirt off my back. I was properly violated. There were hands everywhere. All I had left was a pair of shorts. And I let them. If I'd fought in any way, I don't have any doubt they would have killed me. It was just an absolutely frenzied mob mentality. They were stealing everything they could find. Chairs, tables, sleeping mats. They stole the electric cables from the walls."

The vast steel gates at the entrance were lifted off their hinges and taken away and all the prisoners escaped – "apart from me," says Simpson. "From a prison of 750 men, I was the only one left." The guards caught him trying to leave and locked him up.

"They made me lie on the floor while they looted whatever was left then they left me in the cell. There were murderers and rapists roaming the streets, and the only person left locked up pretty much in the whole of Bangui was me."

He was held in the prison for several hours until his boss, a Swedish national called Erik Mararv, who was born and brought up in the republic, arranged for the judge in the case to go to the prison and sign for his release.

He was taken to Marav's house, confused and shaken. "I was in a bit of shock. My head was still in prison. I couldn't quite come to terms with what had happened. I've had so many bizarre things happen, you'd never believe. This is the second riot I've been in this year."

Last month, the Observer spoke to Simpson at length from his cell in Bangui, as well as to his family, who were trying to understand the enormity of the crimes he'd been accused of, while grappling with the absurdity of it all. His father had taken to calling him "the Beast of Bangui", while his brother, Paul, spent most of the time gently lampooning him. "Only David could go and get accused of carrying out a massacre," he said.

During his imprisonment, he was beaten up three times, by three different gangs. "Because every time I got away, another gang saw me and tried to rob me. I was the only white man in there, so I was just the obvious target. I'm all right but I can't move my neck today. It was the way they grabbed me and pinned me to the ground."

Simpson's manner of telling a story is best described as laconic, but even he admits that it "was quite scary". In the time it took for the rioters to break into his cell, he says "the one thing I did manage to do was eat some bread, so I had something in my stomach".

The previous riot had taken place when he was first accused of carrying out the massacre in the town of Bakouma. "And they came after me and really wanted to kill me." Last week, he was attacked by five men with knives. "And now this."

Technically, Simpson is still a prisoner. "It's just that there's no prison to send me to. It's been completely destroyed. And the nearest one now is 200km away." But he hopes that the case will be closed soon. "The charges still stand, but the judge has said to us and to others that he knows we're innocent. He stayed for a beer last night and said as much again."

"I just wish he'd come home," said his mother, Vicky, from the family home on Friday night. And on his Facebook page, where he'd posted the news of his release, hundreds of supporters and well-wishers had sent messages. "They all say, why don't you just get out of there?"

But he's determined to stay. "I love the country. I love the job. I've seen the worst the Central African Republic has to offer, now I want to see the best… I just haven't dared tell my mum that yet."