Prabagaran Srivijayan was sentenced to death in 2012 for trafficking 22.24 grams (0.8 ounces) of heroin, but has consistently maintained his innocence.
He is expected to be hanged Friday, according to Amnesty International and other activists quoting members of his family.
Trafficking certain volumes of illegal drugs carries the mandatory death penalty in Singapore, except if certain conditions are met for it to be commuted to a life sentence.
"The Singaporean authorities must immediately halt his execution before another person suffers this inhumane and irreversible punishment," James Gomez, Amnesty International's Director for Southeast Asia and the Pacific said in a statement on Tuesday.
Amnesty said the convict's lawyers have raised concerns about the fairness of his trial, including the alleged failure of the authorities "to follow up leads and call on key witnesses that would corroborate his version of events".
Prabagaran's lawyers have also launched a case in Malaysia where the Court of Appeal is considering an application to refer Singapore to the International Court of Justice over concerns about the trial, according to activists.
Both Malaysia and Singapore execute murderers and drug traffickers by hanging, a system which dates back to British colonial rule.
On Wednesday, Singapore anti-death penalty group We Believe in Second Chances also called for Prabagaran's execution to be halted.
"There is no need to rush to hang an individual. Prabagaran's execution should not have been scheduled while his case has yet to be fully heard in the Malaysian courts," the group said in a Facebook post.
Singapore, however, has consistently maintained that the death penalty is an effective deterrent to crime and has rejected calls to abolish capital punishment.
The city-state last November hanged two foreigners -- a Malaysian and a Nigerian -- for drug trafficking after their last-minute appeals were rejected.
Singapore hosts thousands of multinational corporations, many of which have made the city their regional headquarters because of its reputation for safety and incorruptibility.
Published : July 12, 2017
By : Agence France-Presse