Iranian opposition activists have condemned the UN Secretary-General for “legitimising” the Islamic regime after he paid tribute to President Ebrahim Raisi, who was killed in a helicopter crash earlier this month.

In footage posted by Iranian state television, António Guterres can be seen sitting behind a bouquet of lilies, two candles and a photo of Ebrahim Raisi and his foreign minister, Hossein Amir-Abdollahian, who also died in the accident.

The deceased president – nicknamed the ‘Butcher of Tehran’ –repressed anti-regime protests while in office, decades after he ordered the execution of thousands of activists while serving as a judge.

According to the Islamic Republic News Agency (Irna), Guterres was welcomed to Iran’s mission to the United Nations in New York on May 25 by Ambassador Amir-Saeid Iravani.

The secretary-general is said to have expressed his sympathy with, “the Iranian government and nation and the families of the victims.”

Irna added: “The UN chief also offered condolences to the great Iranian nation and government.”

The Norway-based Iranian opposition group Victims Families for Transitional Justice said: "Offering condolences and tributes to a regime notorious for its systemic human rights abuses, support of terrorism, and suppression of dissent, essentially legitimises its heinous actions.”

A book of condolences for Raisi and Amir-Abdollahian was also signed by Amina Mohammed, the UN’s deputy secretary general and Dennis Francis, the president of the UN General Assembly.

Tributes were also paid by members of Neturei Karta, an anti-Zionist Chasidic sect, Irna reported.

Guterres’s visit to Iran’s mission follows a minute of silence held by the UN Security Council in remembrance of Raisi and the others killed when their helicopter crashed in a remote and mountainous region of Iran earlier this month.

“The Secretary-General expresses his sincere condolences to the families of the deceased and to the Government and people of the Islamic Republic of Iran,” Guterres’s spokesperson said in a statement.

At a nuclear security conference in Vienna, Rafael Grossi, the head of the International Atomic Energy Agency – in charge of monitoring Iran's push to develop nuclear weapons – led a minute's silence in remembrance of Raisi.

The deceased Iranian president, a hardline cleric, was appointed deputy prosecutor in Tehran following the 1979 Islamic revolution.

As a member of a “death committee” set up in 1988, Raisi presided over the retrial of thousands of political opponents of the Iranian regime.

In total, around 5,000 people were condemned to death, executed and buried in unmarked mass graves.

When asked about the killings in 2021, Raisi said: "If a judge, a prosecutor, has defended the security of the people, he should be praised... I am proud to have defended human rights in every position I have held so far."

In the 2021 Iranian presidential elections, the former judge won 62 per cent of the vote.

Amid the ensuing Women, Life, Freedom protests, which were sparked after the nation’s notorious morality police killed a young woman, Raisi spearheaded a crackdown on dissent.

Over 20,000 people were arrested when the regime suppressed demonstrations, with around 500 killed.

The UN secretary general’s office was contacted for comment.