When a mother of two can be sentenced to death by stoning on the basis of a disputed confession of adultery and without proper legal representation, there is little reason for faith in the fairness or mercy of Iran's judicial system. But as in the appalling case of Sakineh Mohammadi Ashtiani, the 20-year jail sentences just handed out to seven Bahá'í leaders must provoke an international outcry in the hope that the Iranian government can be shamed into thinking again.

The sentences follow a sham trial in which the accused faced a variety of charges ranging from spreading propaganda against the state and engaging in espionage, to conspiring to commit offences against national security. Unless international pressure can force a change of mind, many of the Bahá'í leaders are doomed to die in prison. The oldest – Jamaloddin Khanjani – is already 77 years old.

Their former lawyer, the Nobel peace prizewinner Shirin Ebadi, who has herself been forced into exile because of her outspoken criticism of the Iranian regime's record on human rights, has said that there is no evidence to support the charges or claims of the prosecutor. What is also clear is that the trial comes after over two years of harsh and illegal detention and unfair treatment which violated not just international norms but also Iranian law.

The five men and two women had already spent eight months in jail before they were charged with any offence. Though Iranian law grants the accused the right to have a lawyer present during the investigation, this right was denied the prisoners.

During two years of incarceration, lawyers working with Ebadi were granted less than two hours with their clients. They had only a few hours to examine the case files, comprising hundreds of pages. In the little time they were granted, they discovered the files were compiled by officials from the ministry of intelligence, despite Iranian law stipulating that such agents "should not be entrusted with the investigation ... of the accused".

The catch-all charge of espionage exposes the reality behind the regime's cruel behaviour. Over the years, Bahá'ís have found themselves accused of being tools of Russian imperialism, British colonialism, American expansionism and most recently Zionism.

But when we learn that Bahá'ís accused of spying for Israel are offered exoneration and the restoration of all the rights of citizenship if they will simply recant their faith, we can see such charges are totally baseless.

The desecration of Bahá'í cemeteries, the demolition of shrines and confiscation of Bahá'í property are unlikely punishments for a band of spies.

The truth behind this sentence is that it is an attempt to decapitate Iran's 300,000 strong Bahá'í community. As members of Iran's biggest religious minority, they have suffered decades of discrimination, harassment and appalling treatment. Most recently, 50 Bahá'í homes were razed in northern Iran, and we know of at least 47 other Bahá'ís currently imprisoned.

Yet the Bahá'í faith, which has its roots in Iran, is a gentle religion which emphasises the spiritual unity of all humankind and builds on the prophets of many faiths, including Jesus and Mohammed. It poses no threat to the Iranian regime. The peaceful, constructive lives led by millions of Bahá'ís in other countries contradict the fears of the Iranian regime. Iran's disregard for its own laws, let alone its human rights obligations, exposes its religious fanaticism.

The foreign secretary, William Hague, has already said he was "appalled" at this "shocking example of the Iranian state's continued discrimination against the Bahá'ís". The US secretary of state, Hillary Clinton, called the sentence a "violation" of Iran's international obligations. Australia, Canada, the European parliament and European Commission, France, Germany and the Netherlands have expressed their concern as have, among others, Amnesty International and Human Rights Watch.

A chorus of condemnation is reminding Iran that it signed and ratified the International Covenant of Civil and Political Rights, in which Article 18 upholds the right to adopt and practice a religion or belief. The Iranian authorities, despite what they pretend, are not deaf to international criticism. We must all add our voices on this latest abandonment of Iran's human rights obligations so the regime hears us loud and clear.