The alleged mercenaries arrested in Zimbabwe are believed to have been on their way to a covert military training camp in Cameroon.
The men were remnants of South Africa's defunct mercenary company Executive Outcomes.
They were planning to join another former Executives Outcomes operative in Equatorial Guinea in an elaborate plot to overthrow President Teodoro Obiang Nguema Mbasogo, which was supposed to have taken place next week.
The coup plot has been denied by the British-based company Logo Logistics, which employs the suspected mercenaries. The company claims they were going to provide security for mining operations in the Democratic Republic of Congo.
But the governments of Equatorial Guinea, South Africa and Zimbabwe have all confirmed the plot.
The plan fell apart when South African law enforcement agencies informed their Equatorial Guinea counterparts of the coup plot some time ago, President Thabo Mbeki's spokesperson said last night.
This led to the impounding of a Boeing 747 aircraft, carrying the 64 alleged mercenaries, in Harare, and of another 15 men in Malabo, the capital of Equatorial Guinea, at the weekend.
Equatorial Guinea's government claims the 15 were the advance party to the group now being interrogated in various Zimbabwe jails.
They said the leader of the advance party was South African Nick du Toit, believed to be a former member of Executive Outcomes.
Flanked by armed guards, he was "invited" to make a public confession of the coup plot in Malabo on Tuesday. Du Toit admitted that the advance party planned to abduct Obiang and force him into exile.
"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 installing the government in exile of Severo Moto Nsa," Du Toit said.
Severo Moto Nsa is an opposition leader living in exile in Spain. He has denied any role in the coup plot.
But Patrick Smith, editor of the London-based Africa Confidential, said on Wednesday that he had evidence to prove that Logo Logistics was linked to a wealthy Lebanese businessman with close ties to Moto.
Although the Zimbabwean government initially suspected it was the target of the alleged mercenaries, Home Affairs Minister Kembo Mohadi said last night the men were on their way to Equatorial Guinea.
He added that Equatorial Guinea's police and army leaders had gone along with the plot against Obiang.
According to security sources, the men were on their way to a covert military training camp in Cameroon, the nearest country on the African mainland to Malabo, which is on the island of Bioko.
A South African intelligence agent said the camp had been used to train and equip opposition groups in Equatorial Guinea for the past six months.
The agent said the alleged mercenaries had all been recruited in South Africa.
"Information gathered has shown that these men were recruited over the past four months and were to head up the training for the Equatorial Guinea opposition forces," the agent said.
Remnants of the notorious Executive Outcomes, which shut down in 1999, are prominent in the coup plot.
Mohadi named Simon Witherspoon, a "known South African mercenary", as the apparent leader of the group and also mentioned Simon Mann, a former British Special Air Service member, as being among those arrested in Harare.
Witherspoon, Mann and Du Toit formed the basis of Executive Outcomes, said an analyst, who requested anonymity.
They men were due to appear in court in Harare on Thursday.