Eight drug-trafficking convicts from several countries were executed by an Indonesian firing squad shortly after midnight on Wednesday, but a Filipina who was on death row with them was unexpectedly spared at the last minute.

Earlier, Jakarta rejected last-ditch pleas from around the world for clemency to be granted the drug traffickers from Nigeria, Australia, Brazil and Indonesia, ordering their mass execution to proceed within hours.

A spokesman for the Attorney General's Office said the execution of Mary Jane Veloso, a mother of two who was arrested in 2010 after she arrived in Indonesia with 2.6 kg of heroin hidden in her suitcase, had been delayed.

He said the delay came in response to a request from Manila after a drug courier gave herself up to police in the Philippines on Tuesday.

People holding a vigil outside the Indonesian embassy in Manila cheered and clapped on hearing the news.

The proposed death penalties were condemned by the United Nations, and have strained ties between Australia and Indonesia in particular.

Hours before the expected executions, crowds gathered in cities across Australia to hold vigils for Myuran Sukumaran and Andrew Chan, holding placards and calling for Australia to respond strongly to its neighbour if the executions proceed.

Security was tight at the prison on an island off the Central Java coast where the executions were to take place, and a dozen ambulances, some carrying white satin-covered coffins, were seen arriving.

NO TIME GIVEN

Indonesian authorities had declined to specify a time for the executions, which were due to take place at a nearby clearing in a forest. But when a group of drug traffickers was executed earlier this year, it was carried out at midnight.

Officials said the prisoners were to be given the choice to stand, kneel or sit before the firing squad, and to be blindfolded. Their hands and feet were to be tied.

Twelve marksmen were assigned to fire at the heart of each prisoner, but only three would have live ammunition. Authorities say this is so that the executioner remains unidentified.

Philippine President Benigno Aquino said on Tuesday that he had made one last appeal to the Indonesian government to spare Veloso, arguing that she could be a vital witness in prosecuting drug syndicates.

Authorities on Monday granted Australian Chan's final wish, which was to marry his Indonesian girlfriend at the prison.

But they rebuffed last-minute appeals from Australia to save the lives of Sukumaran and Chan, who were arrested in 2005 as the ringleaders of a plot to smuggle heroin out of Indonesia.

Australian Foreign Minister Julie Bishop told ABC television earlier: "Should these executions proceed in the manner that I anticipate, of course, there will have to be consequences."

Australia-Indonesia relations have been tested in recent years by disputes over people smuggling and spying. In late 2013 Indonesia recalled its envoy and froze military and intelligence cooperation over reports that Canberra had spied on top Indonesian officials, including the former president's wife.

Indonesia has harsh punishments for drug crimes and resumed executions in 2013 after a five-year gap. Six have been executed so far this year.

President Joko Widodo's steadfastness on the executions, which has strong public support at home, stands in contrast to a series of policy flip-flops since he took office six months ago.