Iran is estimated to have executed 273 people in 2018, the second highest figure in the world, according to a report by the NGO Iran Human Rights.
The news comes as around 1,500 delegates gathered in Brussels for the 7th World Congress Against the Death Penalty.
Speaking to the media at the conference, Mahmood Amiry Moghaddam of IHR said the deaths were confirmed through independent sources, but that two-thirds of them were not announced officially.
IHR verifies all unofficial reports by consulting two independent sources, often eye-witnesses, family members or lawyers.
Although the figure is high, IHR says it represents a 10-year low for the country.
The group says the four leading charges that result in execution in Iran are murder, moharebeh (corruption on earth), drug-related offences and rape.
"Corruption on earth" is the second most frequent charge for the death penalty. Broadly defined, it can be applied to a wide range of offences, including political affiliation, cooperation with the so-called Islamic State and economic corruption.
Moghaddam says it is largely up to the discretion of the Revolutionary Court to decide what crimes can be categorised as moharebeh.
“The due process, which is, in general, violated daily in Iran, is even worse in the Revolutionary Courts," Moghaddam said.
IHR says Iran also has the highest rate of juvenile executions in the world, with six confirmed executions of minors in 2018, including two child brides charged with the murders of their husbands.
In the annual report, IHR urges the international community to cooperate with Iran with the stipulation of “compliance with human rights standards.”
Due to a lack of transparency from the Iranian government, it is difficult for IHR to gauge the actual number of deaths by capital punishment.
This is especially prevalent in the ethnic minority regions of Iran where official reports of execution are lowest.