He recovers and bends his gangly body with a laugh, looking for all the world like a teenager making a home video with friends. Another young man follows him, walking stiffly. Someone in a thin grey suit kisses both men on both cheeks and strolls off-camera.

Dozens of people are milling about. A crowd can be seen held back by barriers, but even the guards look relaxed, standing well back from the two with their hands bound. Two rusty cranes on flatbed trucks are parked on the grass, the ropes hanging from each are rough, tangled with knots and the noose at the end looks amateurish - like a random piece of rope washed up on a beach.

Almost casually someone puts the rope round the awkward youth's neck first, then the second, steps back and the cranes pull up the ropes. The second man's body is still, and the camera stays on the taller one until he stops moving, about six minutes.

The film shows the public hanging of Alireza Gorji, 23, and his friend Hossein Makesh, 22, in July in Broudjerd, Iran. According to official versions of the charges, they were put to death because they had behaved 'immorally'. The truth, according to anti-government campaigners, is that the two men were among increasing numbers of political activists being executed by Iran on trumped-up charges.

'Both these men had been involved in anti-government protests in their home town and everyone who watch the hanging knew this,' said a human rights observer in Tehran.

On Tuesday the UN General Assembly condemned Iran for human rights abuses and the video - filmed by a Revolutionary Guard, smuggled out by opposition activists and seen by The Observer - is rare evidence of Iran's efforts to quell dissent. Amnesty International last year documented at least 94 public executions although many more are suspected to take place in secret - in September the authorities told a lawyer for Valliollah Feyz-Mahdavi, 28, that he had died after a suicide attempt in prison. Feyz-Mahdavi had been arrested for membership of Iran's main opposition - the People's Mojahedin Organistation of Iran.

Tehran has now been condemned on more than 50 occasions by the UN for severe human rights violations.

The Broudjerd video has been obtained by an exiled opposition group - the National Council of Resistance of Iran. At the House of Commons on Tuesday, it will be shown to cross-party MPs to encourage the British government to reconsider what the National Council regards as a policy of appeasing the Iranian regime. The group will unveil documents on the execution of more than 20,000 political victims, including evidence for the involvement of President Mohammad Ahmadinejad.