The final session of the Iran Tribunal investigation into the execution of 20,000 political prisoners in the 1980s opens today in The Hague.

The charges against Iran include crimes against humanity.

In one very bloody seven month period, from August 1988 to February 1989, at least 4,500 people, some of them teenagers, were executed, according to Amnesty International. This works out at an average execution rate of one person almost every hour for over 200 days.

The current regime of President Ahmadinejad refuses to acknowledge these mass killings or provide any redress.

The Iran Tribunal, inaugurated in 2007, comprises leading judges and lawyers from around the world, including former prosecutors at the International Criminal Court (ICC) and at the special tribunals on Rwanda and the former Yugoslavia.

British legal experts on the tribunal's legal steering committee include Prof John Cooper QC and Sir Geoffrey Nice, Gresham professor of law and a former prosecutor at the ICC.

The tribunal was set up after past attempts to get the United Nations to investigate the killings were blocked by Iran's allies in the Human Rights Council.

Most of the people executed were democrats, secularists, liberals, students and left-wingers. They were shot by firing squads or hanged from cranes, usually after prolonged torture and unfair, summary trials. Some women were raped before their execution. The victims were buried in mass graves. Many of the bodies have never been recovered.

The previous session of the tribunal, the Truth Commission in June 2012, heard witness testimonies of floggings, burnings, slashings, nerve constriction, suspension from ceilings and mock executions. The sexual abuse of detainees was commonplace. Prisoners were often kept in solitary confinement in tiny two-square metre cells; and subjected to sleep deprivation and starvation rations. Many were stripped naked and exposed to prolonged extreme heat or cold. Children as young as 11 years old were among those tortured and hanged.

A member of the Iran Tribunal, Professor Payam Akhavan, observed:

"It (the tribunal) is a unique opportunity for the Iranian people to hold those in power accountable for past injustices to build a better future based on the rule of law.

"Instead of being punished, the perpetrators of these heinous crimes have been promoted to senior positions in government; members of the Death Commission sit on the Iranian Supreme Court, in its parliaments and in its Cabinets....without accountability for past crimes, it will be difficult to build a culture of human rights in Iran and to move beyond the present culture of impunity," he said.

Unlike the massacres in apartheid South Africa, Darfur, Srebrenica and General Pinochet's Chile, there has never been any international outrage at the mass killings in Iran. The victims and their loved ones have never had any opportunity for justice and legal redress.

This tribunal is the first time the full scale of the executions and the suffering of the victims has been subjected to authoritative judicial investigation and documentation. For the loved ones of those who were murdered, documenting these horrendous crimes is the first step towards exposing the perpetrators and hopefully one day bringing them to justice.

Repression continues in Iran today, with frequent arbitrary arrests, sham trials, torture, forced confessions and the incarceration and execution of political prisoners.

Since the fraudulent presidential elections in 2009, Amnesty International has documented an intensifying crack down by the Tehran regime. Its alarming report, published this year, is entitled: We Are Ordered To Crush You: Expanding Repression of Dissent in Iran.

Within the last week, three campaigners from the persecuted Baluch ethnic minority community have been hanged, after unfair trials and confessions extracted under torture.

In June, four Ahwazi Arab men, members of another victimised minority nationality, were executed for "enmity against God and corruption on earth." Their families say they were tortured into confessing to the killing of a law enforcement official.

A further nine Arab political prisoners are on death row, awaiting execution. Already seven Arab activists have been killed extra-judicially under torture this year.

The UN special rapporteur on human rights in Iran, Ahmad Shaheed, a former foreign minister of the Maldives, this week presented a report that warned of worsening discrimination against religious and ethnic minorities, women and gay people.

He noted that there had been more than 300 officially reported executions in the first eight months of this year but that the real figure is probably much higher because Iran is now less open about the number of people it puts to death.

Shaheed said that 670 people were executed in 2011, making Iran the country with the world's highest per capita use of the death penalty.

Aside from executions, the Iranian regime stands accused of persecuting political and ethnic dissidents, Sunni Muslims and other religious minorities, trade unionists, students, journalists, lawyers, women's rights activists and LGBT people.