Tangaraju Suppiah was hanged in defiance of a plea by the United Nations Human Rights Office for Singapore to "urgently reconsider" and calls by British tycoon Richard Branson to halt it.

The Asian financial hub has some of the world's toughest antinarcotics laws and insists the death penalty remains an effective deterrent against trafficking.

"Singaporean Tangaraju Suppiah, 46, had his capital sentence carried out today at Changi Prison Complex," a spokesman for the Singapore Prison Service told AFP.

Tangaraju had been convicted in 2017 of "abetting by engaging in a conspiracy to traffic" 1,017.9 grams of cannabis, twice the minimum volume required for a death sentence in Singapore, the spokesman said.

He was sentenced to death in 2018 with the Court of Appeal later upholding the decision, but rights groups have claimed there were several problems with the case.

Human Rights Watch deputy Asia director Phil Robertson said the evidence "was far from clear cut, since he never actually touched the marijuana in question, was questioned by police without a lawyer and denied access to a Tamil interpreter when he asked for one".

He added the hanging "raises serious concerns that Singapore is launching a renewed spree to empty its death row in a misguided effort of deterrence".

Amnesty International deputy regional director Ming Yu Hah said there were "many flaws" in the case and the hanging showed "the staggering failure of Singapore’s stubborn embrace of the death penalty".

'Not anywhere near' drugs  

Singapore authorities have maintained Tangaraju had been given due process and his guilt had been proven beyond a reasonable doubt.

The Home Ministry said "the evidence clearly showed that he was the person coordinating the delivery of drugs, for the purpose of trafficking".

Branson, a member of the Geneva, Switzerland-based Global Commission on Drug Policy, wrote on Monday on his blog Tangaraju was "not anywhere near" the drugs at the time of his arrest and Singapore may be about to put an innocent man to death.

On Tuesday, the ministry slammed Branson, saying the billionaire showed "disrespect for Singapore’s judges and our criminal justice system with such allegations".

In some parts of the world, including neighboring Thailand, cannabis has been decriminalized, with authorities abandoning prison sentences.

Rights groups have been heaping pressure on Singapore to abolish capital punishment, and the UN has said the practice had not proven to be an effective deterrent globally and was incompatible with international human rights law.

Singapore resumed executions in March 2022 after a hiatus of more than two years.

Wednesday's hanging was the city-state's first in six months and 12th since last year.

Among those hanged was Nagaenthran K. Dharmalingam, whose execution sparked a global outcry, including from the UN and Branson, because he was deemed to have a mental disability.