Zimbabwe is to charge more than 60 men said to be mercenaries with plotting to "destabilise a sovereign government," Home Minister Kembo Mohadi has said.
Military equipment was reportedly found on the impounded plane
The men were detained after their plane was impounded in Harare on Sunday.
Zimbabwe has accused the group of planning a coup in oil-rich Equatorial Guinea. A government minister has said the men could face the death penalty.
A lawyer representing the group has met his clients and said they have not complained about their treatment.
"They have not been ill-treated, they are happy and were even saying they are being given too much food in jail," Jonathan Samkange was quoted by AFP news agency as saying.
The men are expected to appear in court on Monday.
In Equatorial Guinea, a man said to be related to the group has publicly confessed to a coup plot on television.
On Thursday, Equatorial Guinea Deputy Foreign Minister Jose Esono Micha asked South Africa for help in the trial of another 15 suspected mercenaries.
From Pretoria, he travelled to Harare to discuss the issue with Zimbabwean officials.
The 15 men - arrested in Equatorial Guinea - were reportedly involved in the same alleged plot.
South African President Thabo Mbeki said Equatorial Guinea wanted South African assistance to ensure the trial of the 15 men was transparent and fair.
"It was a direct request," Mr Mbeki said after meeting officials from the western African state.
The company which chartered the plane impounded in ZImbabwe said they had stopped in Harare en route to Burundi and DR Congo where they were due to provide security services for an internationally run mine.
But Zimbabwe and South Africa both support the government of Equatorial Guinea and say the men were part of a plot to overthrow the president, Teodoro Obiang Nguema.
Harare has accused the men of working for US, British and Spanish intelligence agencies.
History of coups
The group of the 64 detained men consisted of about 20 South Africans, and also Angolans, Namibians, Congolese and one Zimbabwean national, South African Foreign Ministry spokesman Ronnie Mamoepa told the BBC's World Today programme.
Equatorial Guinea and its president have a long history of coups
"We are awaiting the outcome of investigations into this matter, and if indeed it leads to a trial, we will monitor the trial to see if it is fair," Mr Mamoepa said.
Equatorial Guinea's president said South Africa had warned him that a group of mercenaries was heading for his country and he suggested that they had had foreign backing of hostile foreign powers and multinational firms.
A man said to be the leader of the "mercenaries" has appeared on Equatorial Guinea state television to say that they had been part of a plot to remove Mr Obiang and put an exiled opposition leader in power.
"It wasn't a question of taking the life of the head of state but of spiriting him away, taking him to Spain and forcing him into exile and then of immediately installing the government-in-exile of Severo Moto Nsa," said the alleged coup leader, introduced as Nick du Toit, according to a transcript released by AFP news agency.
In Spain, Mr Moto, who led a failed coup bid in Equatorial Guinea in 1997, denied any role in the alleged plot but also launched a blistering attack on President Obiang, who himself took power in a coup in 1979.