Declaring that his unprecedented action was not a military coup, Brigadier Gen Singirok accused Sir Julius's government of "corrupt practices" in hiring the mercenaries from Sandline International.
"As armed forces commander, I believe the strategy of the government is wrong, and I have basically told the Prime Minister and the Minister for Defence that what they have done is incorrect. We don't have a coup. I believe in a very strong democratic country, that the country should be run for the people by the people. It's a matter of principle and ethics," he said.
The army was holding more than 40 Sandline mercenaries, most from South Africa, in custody last night. Brigadier Gen Singirok said he believed their appointment had been a "quick fix solution", and that they would be "repatriated" from Papua New Guinea as soon as possible. "I, as commander, am not going to play the game," he said.
After a cabinet meeting lasting several hours, Sir Julius announced he would not resign, described the brigadier's command as a constitutional breach and said that, while Brigadier Gen Singirok may disagree with the the government's actions, he had to obey orders.
The crisis follows the revelation last month that Sir Julius's government, which is facing a general election later this year, had hired what he described as "foreign military advisers" to guide the army's operations on Bougainville, where rebels have been waging a secessionist war for eight years.
The mercenaries, as they have been more widely described, come from Sandline International, a company which is registered in the Bahamas and with offices in London and Washington.
The arrival of mercenaries in Papua, the first such involvement of an outside private army in the South Pacific, has caused alarm in the region, particularly in Australia, which ruled the country until independence in 1975. John Howard, the Australian Prime Minister, held talks with Sir Julius lasting four hours in Sydney a week ago, to persuade him to end his government's involvement with Sandline. Australia provides a substantial portion of Papua New Guinea's budget in direct aid.
Mr Howard called on the Papua New Guinea government yesterday to resolve the stand-off with its armed forces peacefully and constitutionally. "This is the sort of destabilisation that we feared," he said.
While Port Moresby, the capital of Papua New Guinea, remained calm last night, it was unclear how far support for Brigadier Gen Singirok's ultimatum to Sir Julius extended among the police and the armed forces.
The brigadier claimed that he had the backing of Bob Nenta, the Papua New Guinea police commissioner, but Mr Nenta later dissociated himself from the move.
Brigadier Gen Singirok was appointed army commander about 15 months ago, at a critical time in Port Moresby's lengthy, unsuccessful campaign to bring the Bougainville rebels to heel.
Sir Julius's deal with the mercenaries angered Brigadier Gen Singirok, who interpreted the move as an attack on his integrity as army commander and a vote of no confidence in his forces.