Singapore has hanged a man over a cannabis smuggling plot - despite calls from the United Nations to stop the execution.

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.