‘Making this film has made my life hell in Egypt,” says Mohamed Diab. It was hell to shoot, he says, it was hell to get released, and a fair proportion of the Egyptian public gave him hell when they saw it. But perhaps that’s a sign he achieved what he set out to do.

Clash captures the chaos of post-revolution Egypt in microcosm. It is set entirely in the back of a Cairo police van, during a long, hot day in 2013, marked by violent street clashes between supporters and opponents of Mohamed Morsi’s then-newly elected Muslim Brotherhood government. As the film progresses, the van fills with a cross-section of society: rebels, Islamists, mothers, children, journalists and ordinary youths caught up in the unrest. Inevitably, things kick off in this pressure cooker, but new allegiances and connections also emerge – along generational, gender and religious lines, even between football supporters and dog-lovers.

Diab was something of a symbol of the Egyptian revolution even before Clash. The 39-year-old was established as a writer of some of the most commercially successful movies in Egyptian history, including the crime saga El Gezira (The Island) and its sequel, which he says is like “an Egyptian Godfather”. His debut as a director, with 2010’s Cairo 678, controversially broached the topic of sexual harassment – and dealt with Mubarak-era societal malaise. Released a month before the Tahrir Square protests began, it was seen by many as a harbinger of the revolution.

Diab endorsed and participated in the protests “from day one”, he says. “I’m not particularly an activist – everyone who was involved in the revolution is an ‘activist’, if you want to call them that. I was just privileged enough to be someone that was known, who could go on TV and talk about it.” He put his career to one side and spent the next three years making media appearances, meeting political leaders, “trying to change the world on the ground”, he says. “But eventually, I felt I had more power as a film-maker.”

Clash, which Diab cowrote with his brother Khaled, captures the chaos and intensity of that time on a surprisingly epic scale. It is not just a claustrophobic chamber piece; the camera regularly peers through the van’s windows to capture what is going on outside, too: running battles with protesters, police charges, a sniper attack and a full-on street riot that approaches levels of surreal abstraction with its maelstrom of fireworks, gunfire, chanting, clattering metal and green-light beams from laser pointers. “Another name for this film would have been Hysteria,” Diab says.

Engineering that hysteria for the camera entailed its own risks. Film-makers in Egypt have to submit their scripts for approval to obtain permission to shoot on the streets, but knowing how the authorities would react to his story, Diab omitted contentious aspects. Staging the riot was a clandestine operation. “We planned that day for a month,” he says. “It was like a flashmob: everyone’s gonna show up and we’re going to shoot the scene until someone stops us. We knew that after the first firework went off, the police were going to storm the place and we might get attacked by the neighbourhood, and this is exactly what happened. One of the extras got stabbed, my producer got hijacked in his car. It was crazy. Just like you see in the film.”

There were other headaches: “The extras were not trained; this was the first time they had ever acted. When I said ‘action’, it was easy, but after I said ‘cut’, it took 15 minutes to calm them down. And when we said, ‘Pretend to hit each other,’ they actually hit each other.” And that was on top of the day-to-day challenges of confining all of his actors in a sweltering metal box, then trying to direct them from the outside.

Diab’s next circle of hell was getting the film released. He calculated that Clash would only get past the Egyptian censors with the “protection” of an international profile. That part of the strategy succeeded: Clash opened the Un Certain Regard selection at Cannes last year and received unanimously positive reviews, including a typed letter of encouragement from Tom Hanks. “Few Americans see Egypt as being anything more than terrorists and pyramids,” he wrote. “Your film will go great lengths to enlighten many.”

“Yet the moment it got premiered and got great reviews, Egyptian TV started attacking me,” says Diab. “They ran this 10-minute piece about me and the film, and how it was portraying Egypt in a bad way; insinuating I was a spy, somehow. And this continued. Many newspapers started attacking me.” He was essentially the victim of a propaganda campaign.

Abdel Fattah el-Sisi’s government, which deposed Morsi’s Muslim Brotherhood, started leaking personal information about him, Diab says. They cast suspicion over his former career in banking (he worked for Citibank for two years before becoming a film-maker) and his American education (he studied screenwriting at the New York Film Academy for a year). A week before the film’s intended release, a distributor pulled Clash from its cinemas. Diab reckons somebody “spoke to him”. “Part of the plan was not to ban the film and make an international scene, but to kill the film in a way that no one would hear about it,” he says. In response, Diab posted a message on Facebook explaining what had been going on and attaching Hanks’ letter. The post went viral and the public mobilised to support the film.

Oddly, when they saw the film, many of those who had supported its release were disappointed by its non-partisan view. “The people thought: ‘If the government is fighting the film, it must be a film against the government,’” Diab says. “When they saw that the film humanised everyone, even the police, they started attacking it, too.” He likens the situation to making a film in Britain just after the second world war and sympathising with the Germans. “We used to say, if it got attacked equally then we made the film we wanted to make. And that’s exactly what happened.”

Diab describes Sisi’s military regime as “10 times worse” than the Mubarak era, but Egypt is stuck with him for the foreseeable future. The former defence minister recently received the endorsement of Donald Trump’s White House, and few Egyptians have the appetite for another revolution, which means that massacres, human rights abuses and media repression are here to stay.

“I’m someone who was committed to the revolution 100%, but you have to know, dictatorship has some advantages,” he says. “Dictatorship shuts up everyone; so there is no division. The only problem with this is, we never even see our differences. We realised we were different once we threw off the dictatorship, and it’s as if we were discovering ourselves.”

Egypt’s newfound democracy became a way of choosing another dictator, he says. The military regime now controls every sector of the economy, including the media. Egypt is the cultural powerhouse of the Arab world in terms of film and television entertainment and, according to reports, the military now owns much of it outright. There are more imprisoned journalists than ever before. “Anyone who says anything that is against the government is jailed, vilified, attacked, fired,” says Diab. Satirists, film-makers, writers and artists, including friends of his, have had to leave the country. Diab himself is in a slightly different position: “In a way, I’m a high-profile character; they’re never going to touch me. But now everyone knows that working with me is a hassle. So I haven’t done anything since Clash.”

Diab is also planning to leave the country. After four years of political activism, he wants a break. He hopes to work in the US next (his wife and co-producer, Sarah Goher, is Egyptian-American). He is just finishing the third draft of a big sci-fi film he says, to do with global warming and inter-generational conflict. “I have always dreamed of making films in Hollywood,” he says. “The only thing is, am I going to make films about what I want to say or just be a director for hire? I respect everyone who does that, but my superpower is passion, so I need to be passionate about something.”

He is optimistic about his own prospects, but far less so for those of his country. “I expressed my feelings in the film,” he says. “There’s a car that’s going in the wrong direction, and things don’t end up well. This is how I see things in Egypt for sure. No matter how good a driver you are, if you take the wrong turn, you are not going to reach your destination and you are probably going to end up in a very bad situation.”

Clash is released in the UK on 21 April.