Iran is executing people for alleged drugs offences at a record rate, with at least 488 hanged so far this year, according to Amnesty International.
The figure marks a nearly three-fold increase from 2009.
"To try to contain their immense drug problem, the Iranian authorities have carried out a killing spree of immense proportions," said Ann Harrison, a Middle East director at the human rights organisation.
Most at risk are those from poor communities, ethnic minorities and foreign nationals, in particular Afghans, as many as 4,000 of whom "could be on death row for drugs offences", said an Amnesty report, due to be released today.
Most executions follow "grossly unfair trails" and the accused, their families and lawyers often receive "little or no warning" that executions are imminent.
There have been secret mass hangings at one prison near the border with Afghanistan, where a long beam in a passageway between the cells and the visiting room is slung with as many as 60 nooses.
About 81 per cent of at least 600 executions up to the end of November were for drug-related offences, Amnesty said. It urged Iran to abolish the death penalty for all crimes but said halting the hanging of drug offenders should be the first step and would drastically reduce its execution rate.
The country is second only to China in the number it executes.
In battling drugs smuggling from Afghanistan, Iran suffers a costly spillover effect - it is flooded with cheap heroin and opium and has the world's highest addiction to both.
Most of the Afghan drugs are bound for European markets and Iran has said that if it reduces hangings, the West will pay the price.