Singapore has executed Australian drug smuggler Nguyen Tuong Van, rejecting requests from Australian Prime Minister John Howard to spare the condemned man's life.

While death penalty opponents held prayer vigils in Singapore and Australia, Nguyen Tuong Van was hanged at Singapore's Changi Prison at dawn on Friday.

Nguyen had admitted attempting to smuggle 396 grams of heroin in 2002 from Cambodia to Australia via Singapore.

Copy of photo of Nguyen Tuong Van, placed in front of his coffin at funeral parlour, Friday

The 25-year-old naturalized Australian citizen claimed he was working for a Sydney drug syndicate to help pay off debts of around $22,000 incurred by his twin brother, Khoa.

Nguyen's impending execution sparked an uproar in Australia, which abolished the death penalty in 1973. But despite repeated pleas by officials from the prime minister down for him to be spared, Singapore refused to make an exception to its mandatory death sentence for serious narcotics crimes.

Australian Foreign Minister Alexander Downer conceded Friday that Nguyen, an ethnic Vietnamese, had committed a serious crime, but said he was sorry to see him die for it.

"You know, I feel very sad about this," he said. "Van Nguyen did the wrong thing, trafficking drugs, and countries in the region are right to stamp on drug trafficking and be tough and ruthless in dealing with drug trafficking. I'm sad that we've not been able to save his life."

Sinapan Samydorai is president of Think Center, a private Singapore human rights organization. He says the death penalty does not deter people from either using drugs or smuggling them into the country.

"Does it really serve the purpose? By hanging Van, does it solve the problem of drug addiction or drug related problems in Singapore? The answer is obviously no…So it's a very disproportionate way of dealing with the issue, by killing a person," he said.

Singaporean Prime Minister Lee Hsien Loong replied by noting that Nguyen was caught was a very large amount of heroin, and said the convicted man had exhausted Singapore's legal process.

"We take a very serious view of drug trafficking," he said. "The penalty is death. In this case it was an enormous amount of drugs which were being trafficked - nearly 400 grams of pure heroin, which is the equivalent of 26-thousand doses of heroin. The man was charged, convicted, [an] appeal dismissed. He put up a clemency petition. The petition was considered. All factors were taken into account including petitions and letters from Australian leaders. Finally, the government decided that the law had to take its course."

Amnesty International says Singapore, with a population of just over four million people, has the highest per-capita execution rate in the world. Since 1991, more than 400 people have been executed, the majority of them for drug trafficking.