Tangaraju Suppiah was hanged at dawn on Wednesday after being found guilty of conspiring to smuggle a kilogram of cannabis into the country from neighbouring Malaysia.

Protesters previously claimed that the 46-year-old, who denied involvement in the plot, had been convicted on weak evidence - a claim denied by authorities in Singapore.

Relatives and activists had previously sent letters to Singapore's President Halimah Yacob to plead for leniency.

His sentence also drew the attention of the United Nations Human Rights Office, which called on the government to "urgently reconsider", while British entrepreneur Richard Branson described the case as "shocking".

Transformative Justice Collective (TJC), a local group that had also campaigned against Tangaraju Suppiah's death sentence, said he had been hanged in Changi prison on Wednesday.

The Anti-Death Penalty Asia Network condemned the execution as "reprehensible".

"The continued use of the death penalty by the Singaporean government is an act of flagrant disregard for international human rights norms and casts aspersion on the legitimacy of Singapore's criminal justice system," the statement said.

Singapore's anti-drug laws are some of the strictest in the world - with those guilty of trafficking more than 500 grams of cannabis potentially facing the death penalty.

Last year, 11 people were executed for drug offences in Singapore.

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Malaysia, a neighbouring country, abolished mandatory death penalties earlier this month.

However, Singapore's government maintains the death penalty is necessary to protect its citizens and all those executed have been accorded full due process under the law.

Authorities also say their strict laws act as a deterrent effect - and say a study shows that traffickers often carry amounts below the threshold that would bring a death penalty as a result.

Although Tangaraju Suppiah was not caught with the cannabis, prosecutors said phone numbers traced him as the person responsible for coordinating the delivery of the drugs.

He had maintained that he was not the one communicating with the others connected to the case.

An application filed on Monday for a stay of execution was dismissed without a hearing on Tuesday.

The case drew criticism from British billionaire Richard Branson - a long-time campaigner against the death penalty.

In a blog post shared prior to the execution, he wrote: "Tangaraju's case is shocking on multiple levels.

"Singapore has a long and troubled history of executing drug offenders, following mandatory sentencing laws that proscribe the death penalty for certain threshold amounts of drugs.

"The country's government has repeatedly claimed that its draconian laws serve as an effective deterrent of drug-related crime.

"However, Singaporean authorities have repeatedly failed to provide any tangible evidence for that assertion."