The first day of April is traditionally a day of fun and laughter in Britain. For most Iranians it is Sizdah-Bidar – a time of family, picnics, and outdoor celebration.

But for Iran's seven Baha'i leaders, it has another meaning: 10,000 cumulative days of unjustified imprisonment, with no prospect of release until 2028. Shut away from the world, their "family" is now the hundreds of other prisoners of conscience that languish in Iran's prisons. The seven are distinguished for their services to society, not criminality, yet they now survive in cramped, pestilential conditions, lacking essential medical care. Their suffering is emblematic of the human rights crisis in Iran. An international campaign is being launched to raise awareness of their plight.

Iran's pattern of quashing political dissent is engraved on the world's consciousness. Few will forget the shooting in 2009 of Neda Agha-Soltan – an act that symbolises the state's ruthless hold on power. But the oppression of those who do not even challenge the leadership, who are deprived of basic rights and ask merely to be treated as Iranian citizens, is no less significant. I urge people everywhere to take note of the deplorable treatment of the 300,000 Baha'is in Iran. Dr Heiner Bielefeldt, the UN special rapporteur on freedom of religion and belief, recently described it as among the most "extreme manifestations of religious intolerance and persecution" in the world today.

I have many Baha'i friends. They are all kind and considerate – not subversive spies or "others" as Iran's state-controlled media would have you believe. The Baha'i faith was founded in Iran itself, and Iran's Baha'is are an integral part of that country's society. Their contributions are significant too; they established some of Iran's first schools for girls in accordance with their belief in the equality of men and women.

Yet, since the birth of the religion in the mid-19th century, Baha'is have been vilified and oppressed. Around 20,000 were put to death in its early days. After 1979, persecution even became enshrined in state policy.

Iran's state machinery now attacks the Baha'is at every level. Their leadership has been dismantled, access to higher education is denied, and business licences are revoked. Baha'i-owned shops are sealed or burned to the ground, cemeteries are desecrated, homes are raided and property is confiscated. More than 500 have been arrested since 2004. Even their efforts to educate their own youth were declared illegal – a tactic specifically designed to render the whole community's existence unviable.

The Canadian senator Romeó Dallaire's recent analysis of the situation makes chilling reading. Noting the disturbing similarities between Rwanda and the escalating persecution of Iran's Baha'is, he sees it as nothing less than a "slow-motion rehearsal for genocide".

It is high time for Iran's leaders to be called to account for their actions. To release the seven Baha'i leaders – whose only "crime" is tending to the needs of their community – is an essential start if the government is to abide by recognised standards of human rights and due process. Failure to do so only heightens fears that Dallaire's predictions may come true – an indictment not only of Iran, but also of the world's inability to avert yet another human rights tragedy.

• Kamran Hashemi writes for the website United4Iran in London