Almost 700 people are believed to have been executed by Iranian authorities from 1 January to 15 July, according to Amnesty International.
Amnesty said it was an unprecedented spike in executions - and the equivalent of three people per day.
Amnesty International Ireland Executive Direction Colm O’Gorman said the figure paints "a sinister picture of the machinery of the state carrying out premeditated, judicially-sanctioned killings on a mass scale.
"If Iran's authorities maintain this horrifying execution rate we are likely to see more than 1,000 state-sanctioned deaths by the year's end," he added.
Amnesty International is opposed to the use of the death penalty unconditionally and in all cases.
However, it said that death sentences in Iran are particularly disturbing because they are "invariably imposed by courts that are completely lacking in independence and impartiality".
Trials in Iran are deeply flawed, detainees are often denied access to lawyers, and there are inadequate procedures for appeal, pardon and commutation, it said.
Amnesty said the reasons behind this year's surge in executions are unclear but the majority of those put to death in 2015 were convicted on drug charges.
The human rights organisation also noted that executions in Iran did not stop during the holy month of Ramadan, in a departure from established practice.
Meanwhile, Saudi Arabia carried out its first execution in five weeks today after a pause for Ramadan.
Sayir al-Rasheedi was found guilty of fatally shooting two Saudi brothers in a dispute, the official Saudi Press Agency reported, citing the interior ministry.
Authorities carried out the sentence against him in the Qassim region.
The latest beheading brings to 103 the number of executions in the kingdom so far this year, a sharp increase on the 87 recorded during the whole of 2014, according to AFP tallies.
This year's figure is still below the record 192 which Amnesty International said were carried out in 1995.
Human Rights Watch has accused Saudi authorities of waging a "campaign of death" by executing more people in the first six months of this year than in all of last year.
Echoing the concerns of other activists, the New York-based group said it has documented "due process violations" in Saudi Arabia's judiciary that make it difficult for defendants to get fair trials even in capital cases.
Under the conservative kingdom's strict Islamic sharia legal code, drug trafficking, rape, murder, armed robbery and apostasy are all punishable by death.
The interior ministry has cited deterrence as a reason for carrying out the punishment.
