Director Mohamed Diab, 38, told reporters he was afraid of the blowback from back home where authorities are accused of an increasing crackdown on critical voices.
"More and more artists are going to jail. Definitely I fear it. I don't know what is going to happen," said Diab, in the French Riviera town that has become the capital of world cinema for 12 days.
His movie opened the sidebar Un Certain Regard competition in Cannes.
"In Egypt now any individual can see the film, think it's provocative and just file a lawsuit against you and he can put you in jail."
Diab, whose last film "Cairo 678" portrayed sexual harassment on the streets of the Egyptian capital, said he was so "paranoid" filming his latest work, "Clash" that he stopped talking to his revolutionary friends or mentioning politics on social media.
The movie is set in 2013, just after the Egyptian military toppled the deeply unpopular Muslim Brotherhood government of Mohamed Morsi two years after a street revolution ousted president Hosni Mubarak.
In the chaos, military police throw reporters, Morsi supporters and those celebrating his toppling (who are technically on their side) into the back of the van where the entire film takes place to claustrophobic effect.
Whether trying to avoid getting their only cellphone confiscated, sharing water or staying alive as bullets fly and mobs rage outside, the cross-section of Egyptian society has to work together to survive in a very enclosed space.
"I wanted to show everyone's point of view, not only mine. So the car has like 25 people inside it, and everyone of them represents a lot of people that I saw. Real people," said Diab.
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"It's just like craziness, it's just like doomsday, the end of the world, and people see it through the eyes of people stuck inside a car who go through all that action and craziness and mayhem," said Diab.
'GOLDEN AGE HAS PASSED'
He said that filming "Clash" in parallel with the real-time chaos in Cairo caused "unbelievable mayhem" with some actors fighting in real life or passers-by fleeing as they thought they had happened upon real scenes of violence.
Five years after the revolution ousted Mubarak, Egypt is under a government accused of cracking down on activists, bloggers, lawyers and journalists.
Hundreds of pro-Morsi supporters have been killed and thousands imprisoned.
"I remember... the people gathering on one goal and everyone was accepting the other," he said, describing the "utopia" of the first heady days of the 2011 revolution.
"Now after five years it's the opposite, every group hates the other group."
Diab, who took part in the revolution and then protested against Morsi, said his film gives no easy answers.
"No-one is good and no one is bad, we're all shades of grey."
His film sums up the situation in one line: "The golden age of the revolution has passed."