The Zimbabwean authorities say 70 alleged mercenaries have been put in leg irons round the clock after a plot was uncovered to free them from jail.
The men say they are merely mine security guards
A prosecutor said they had received information of a plan to airlift the men from the high-security Chikurubi prison were they are being held.
They are accused of plotting to stage a coup in Equatorial Guinea.
But the men say they were travelling to the Democratic Republic of Congo to work as security guards.
The group have been held since their aircraft was detained at Harare International Airport in March, when it stopped to refuel and pick up military equipment.
They are mostly former members of South Africa's apartheid era military forces.
They are accused of being contracted by Equatorial Guinea opposition leader Severo Moto to stage a coup against President Teodoro Obiang Nguema.
The Zimbabwean authorities have begun legal moves to have them extradited to Equatorial Guinea, where another group is being held on suspicion of being involved in the same plot.
Human rights groups say they believe at least one of the suspects held in custody in Equatorial Guinea has been tortured to death.