Caspian Makan has been run over by the blind, careering juggernaut of history. Just five months ago his girlfriend was killed on the streets of Tehran, one of some 80 deaths reliably reported during the tumultuous demonstrations that followed the disputed presidential elections. Most victims' relatives and friends have grieved in private – but Neda Agha Soltan, Caspian's girlfriend, died live on phone camera, an almost unbearable 90-second sequence that turned her into an icon. Uploaded on to the internet, within hours her face became the face of protest.

But symbols destroy lives. In the days and weeks that followed, Caspian has lost not only the woman he was planning to marry, but also his country, his family, his friends and his career. Anyone and everyone who had anything to do with Neda's death are now toxic to the Iranian government. Members of her family have been bullied, threatened and even detained. The doctor who is caught on camera trying to save her life is now exiled in Britain. The music teacher who was with her when she died has been rolled out on Iranian television, patently required to deny what he saw: that Neda was shot by a member of the religious militia.

And Caspian disappeared. In the days after her killing, he spoke out on foreign satellite stations and then vanished. Finally it was confirmed he was in the notorious Evin Prison in Tehran – the frightening symbol of the Shah's oppressive regime smoothly transferred into the hands of the Islamic Republic's secret police. He was held for more than two months, some of that time in solitary confinement. In September he was released on bail pending trial – perhaps being prepared for one of the extraordinary show trials that have been broadcast on Iranian TV over the past months, in which leading supporters of the opposition have been obliged to recant their actions. Urged on by family and friends, Caspian decided he had to escape.

We meet just days after he got out. He is in hiding and for now does not want people to know where. There is a tangible fear of the reach of the secret police in the Iranian exile community. His temporary home is an empty flat in an anonymous block in the suburbs of a Middle Eastern city he does not know.

He does not look like someone who has just spent five traumatic days escaping his homeland, eight hours of it on foot across remote mountains. Trim and elegant, he greets us with a gentle smile. Tea is offered, though not a chair because there isn't one in the flat. His attempts at pleasantries do not last long. The deep dark circles etched under his eyes betray him.

Neda and Caspian in Turkey.

The whirlwind has consumed everything except the few belongings he could smuggle out of the country – not least a small computer drive. He fires it up and there are his photos of Neda – the beautiful smile that has circled the globe suddenly becomes his private image of love again. Here is the man she was smiling for. He touches her face on the screen.

Their story begins as a classic holiday romance: they were on the same tour group to Turkey in April this year. He talks of Neda as a bright woman, frustrated with life. Turkey is a tantalising place for young Iranians; one of the few places Iranians can visit without a visa. It is a Muslim country but a secular state. There are no headscarves in Caspian's photos of Neda on the holiday.

His voice drops to a whisper as he remembers their first holiday conversations. She was religious and had studied theology and Islamic philosophy at university in Tehran. She'd only been there for three terms when she realised she did not recognise her God in what was being taught. She had married quite young and divorced, which now made getting a job difficult. She loved travelling, and was trying to get a job as a guide in Turkey.

Caspian told her of his life as a professional photographer, that he was also divorced, but they agreed not to talk about previous relationships. They were starting again. She wasn't interested in a casual relationship, and he at 38, 12 years her senior, knew he was serious about her. But both were nervous – a holiday romance, after all.

"For 10 or 12 days [afterwards] we didn't see each other and we had no contact because we wanted to work out how we really felt about each other. We chose a day to meet and agreed that, on that day, we would decide if we really wanted to be together. I knew I wanted to be with her and when she arrived, I knew she felt the same way. I was so happy."

They were not formally engaged, but that was the plan. They even had tickets for a trip back to Turkey together – Istanbul, for the last week in June. The private plans of a private couple. But by then Neda was dead, and Caspian in prison.

What came in between were the presidential elections in June. Neither Caspian nor Neda was very political. All their adult lives the elections had been between a carefully approved range of candidates. Previous dreams of reform had curdled on the realities of a political system dominated by a religious elite. Now it was a face-off between President Ahmadinejad and a former hardline prime minister, Mir Hussein Mousavi. Neither Caspian nor Neda intended to vote.

Then – about a fortnight before the election – something changed. Perhaps it was the televised debates between the candidates that were broadcast in the 10 days leading up to election day. Ahmadinejad, a far more charismatic speaker in public, came across as both aggressive and defensive on the small screen. Unlike the soporific staged affairs of US elections, this was clearly deeply personal. Ahmadinejad attacked Mousavi for allowing his wife to be seen with him in public. Highlights were uploaded to YouTube. Iranians gleefully replayed Mousavi's retort: "This man looks straight into the camera and lies."

Whatever the reasons, Mousavi, no liberal, suddenly acquired a broad appeal. As a friend of Caspian's, who has also had to escape into exile, told us: "I used to think that not voting was a 'no' to the system. This time, I felt, was different. I was encouraging everyone to vote. We just musn't let Ahmadinejad's administration back in with its repression. It wasn't a vote in support of any candidate – it was a protest vote."

The "green movement" was born in those days before the election, and the streets took on a carnival atmosphere. Neda was infected by the excitement. Caspian was not. "I used to tell her 'You are no fan of Mousavi.' She said 'You're right, I am not, but I like his followers. They are asking for their rights. It's not just about one man.'"

It is impossible to say what the election result really was. Certainly the Ahmadinejad victory was announced with unusual haste. Also certain is that hundreds of thousands of Iranians felt they had been cheated. And when they took to the streets they were met by a force assembled from Iran's various security services – uniformed "robocops" with batons, riot police, Revolutionary Guards, and most dangerous of all, the religious militia, Basiji, sometimes in plain clothes. The result, for a few deadly days, was street war. It was pretty one-sided.

The protesters were unarmed, except for stones they picked up off the streets. And camera phones. As we have tried to piece together what happened over those days for our documentary, we have assembled an astonishing mosaic of images – controversial events are covered not by one camera but by 10. Plainclothes police shooting from rooftops are seen from multiple angles – often accompanied by a commentary of shock and outrage. This is citizen journalism, then uploaded on to the web for all to view. The images are impossible to source but as raw data of record they make the task of silent repression almost impossible. Never again will the events of a Tiananmen Square go virtually unrecorded on film. Neda's death was just one of these sequences.

Caspian's voice is strong when he talks about the elections or the politics. Now it drops to a whisper. "She joined the protesters from the beginning. She was very brave and strong. That worried me, to be honest. I didn't want her to get hurt. I asked her to stop going to the protests. I thought she might get arrested or something else might happen to her. But she was only thinking of her goal – democracy and freedom for Iranians."

Neda attended virtually every demonstration – some with her mother, some even with Caspian.

Protesters struggle to save Neda after she has been shot. Photograph: AP

He is deep within himself as he remembers how beautifully she used to explain that everyone should be there. They quarrelled about it. "She said, 'You support me in everything I do, why not this?' I said, 'You don't understand these people. What happens if they catch you?' She said, 'It's not important, Caspian. It's my duty.' She said: 'Caspian, let me tell you the truth. I think that under the circumstances we now have, we're all responsible. Even if we'd had a child, I'd carry my child to these demos on my back.' That's when I realised I couldn't prevent her from going."

On the day of her death, Caspian was out with his camera in another part of the city. "I was taking pictures of the protests and the protesters that day. It was hard to take pictures as the security guards were beating up protesters. I used my mobile's camera when I couldn't use my big camera. It was six to seven in the evening when I started seeing people get shot and injured. I thought of Neda a lot. I was very worried for her. I wanted to call her but the mobile phone system had been disconnected and I couldn't contact her at all. I didn't sleep that night. The terrible scenes were going through my head. I was sitting in front of my computer, looking at the photos I had taken. Around six in the morning my mobile rang. It was Neda's number. But it wasn't her. It was her sister. She said, 'Caspian, Neda is gone!' I didn't understand what she meant. I couldn't believe what she was telling me."

By then the world was beginning to take in the images of her collapsing to the ground, blood coming out of her chest, supported by two men. The last shot most can bear to watch shows her face, beautiful and pale, before blood spills across it, with desperate voices pleading with her. "My child! Neda! Stay with us!" Elsewhere on the web we found further evidence: after the shooting, the crowd confronted the apparent assassin – a member of the religious militia – half naked in front of the phone cameras. A citizens' arrest was pointless but his ID card was taken as evidence and put online.

Shia religion is steeped in the mythology of martyrdom – Iran's Muslim identity has always been defined by sacrifice. The authorities understand this. When the images of Neda led the news across the globe, they moved fast to kill the story.

From contact with her mother and sister and from Caspian's account, we have put together what followed. The family were allowed to bury her but only in a part of the cemetery set aside for the bodies of protesters. They were forbidden to hold any kind of wake – none of the local restaurants, halls or mosques were allowed to accept them. Meanwhile, on television, senior police officers were blaming the violence on terrorist elements, saying that government forces had not been issued with firearms. Distraught, furious at what he was hearing and racked by nightmares, Caspian Makan felt he could not stay silent. He gave interviews over the phone to BBC Persia, Al Jazeera and the Persian-language stations based abroad, in which he described, in brief, what had happened.

His friends begged him to leave the country, but Caspian refused. "I did not want to do this. I was not able to. I could not leave Neda's home. I could not be far away from this movement. I was past caring." On 26 June, six days after her death, the police surrounded his house with snipers on nearby rooftops. "I was at home when they rang the doorbell. They took the whole archive of my work, my editing tools, my documents, all the 10,000 photos I had collected to publish one day. Most of this work is of historical sites in Iran and nature photography.

"They told me they were taking me to Evin prison. They took me to a prison cell. Neda's grave number was 32. The grave next to that was number 34, my cell's number. I didn't want to come back after they took me. I wanted them to kill me as well."

He spent almost two weeks in solitary confinement. He was blindfolded whenever he was moved around. But he could hear. "There were the sounds of beatings, the screams. Sometimes the cries and screams of those kids could be heard. I believe they were mostly young voices. Sometimes I got a glimpse of them through the cell door.

"It was like an examination hall. They seated the youngsters on these chairs. They wanted them to write down everything they did during the demonstrations. I can't say precisely but I think they brought in about 400-500 people there a day. It was so crowded they even had people seated in the showers."

For interrogation, he was transferred to another cell. They sat me down on the ground facing the wall. "There were two or three of them. I never saw them. Only once I saw the shoe of one of them. It was very pointy and shiny. Their line was that Neda was a member of a group opposed to the Islamic Republic of Iran. They insisted on saying that Neda and I were members of a group with plans to cause these events."

Next they suggested that Neda was on a suicide mission – that she had knowingly gone to her death to undermine the state. Caspian could see it was all a formality. "They weren't serious. It was pretty clear that they themselves didn't believe the accusations they were making." What was clear was the damage they felt Neda's death had dealt the Islamic Republic and that he had made it much worse by speaking out.

Then they changed tack. "They said 'The Iranian government is proud of you.' They brought me ice cream and biscuits. Then they wanted me to return to my cell. I went back feeling a little relieved. I thought, OK, let me turn off my light. It was like a searchlight shining straight on my face. Then I realised there was no switch to turn it off."

And so it went on. "They had me on a chair by the wall. They hit me. They told me I was certain to be executed." Over the weeks different versions of what happened were proposed to him but he rejected them all.

After a number of interrogations it went quiet for a week. He was transferred to a new cell for about 40 days but not interrogated again. In one final session the guards admitted neither he nor Neda was part of any political group – "but that I had broken the law by talking". As pressure built – Neda's family was demanding his release, as was Amnesty and other international organisations – they finally let him out on bail. He remains charged with "conspiracy to overthrow the Islamic Republic of Iran". The families had to come up with $100,000 and the deeds to his father's house.

On his release, he spent every morning at Neda's grave. He went early to avoid the security police that hung around the site. "Neda loved sunrise, so I went early to be alone with her then. When the sun came up, people started arriving. It has become like a pilgrimage site."

Everyone was telling him to get out, that he would be arrested again. But it was difficult. "I didn't want to leave. For one, I believe this movement has not died out, and will never die out. But when I saw the constraints I was under, that they had me under constant surveillance, and that I had to keep silent, I really couldn't stand it."

The journey out was traumatic – organised by professional smugglers. He was ill and alone. At one stage he had to cross a mountain pass on his own. It took eight hours of steep climbing.

Caspian looks up. He is relieved to have come to the end. "As I left Tehran, I was looking around at the good people of Iran, who are kind and patient. They looked so weighed down. I am 38 years old. I always will love Iran. It was so hard – I was leaving Neda's resting place. I still cannot believe it. I think I will see her again."

Meanwhile he has a mission. He wants to stage an exhibition in her memory. This quiet dignified man will not let go. "Now I have left Iran, I can cry out. To break the silence."

'An Iranian Martyr', directed by Monica Garnsey, will be broadcast on BBC2 on Tuesday, 24 November, at 9pm.