All five - now aged between 57 and 67 - received the death penalty in May 2010.

"The men can be executed at any moment," said Mella, who has rallied against the use of the death penalty across the globe, particularly on the mainland.

"Hong Kong does not have the death penalty, so it should try to convince the Vietnamese government not to kill the men."

Between 2003 and June last year, at least 75 Hong Kong citizens were sentenced to death or executed abroad, according to Amnesty International figures.

Connie Chan Man-wai, a senior campaigner with the rights group, said the drive to attract support would continue for two weeks. The petition will then be taken to the Vietnamese consulate in Wan Chai.

One of the convicted Hongkongers, 67-year-old Ngan Chiu-kuen, had been ill-treated by the prison authorities and denied access to medical services despite his failing health, the group said.

Mella, who first came to Hong Kong in 1974, is a well-known human rights activist. He first took up the cause of the city's homeless and impoverished, at one point joining a squatter camp in Diamond Hill.

Since then he has championed a variety of causes with some success. He is particularly known for helping boat families in the former Yau Ma Tei typhoon shelter in the 1980s, and as a tireless campaigner for right of abode for cross-border families.

He once went without food for five days in support of seven men who were jailed over a fatal fire in Immigration Tower that started during an abode-seekers' protest in 2000.

In 2011 he was denied a visa to visit the mainland, a decision that Mella put down to troubled relations between Beijing and the Vatican.