The Boeing 727-100 was detained at Harare's main airport on Sunday night after its owners allegedly made "a false declaration of its cargo and crew", said Home Affairs Minister Kembo Mohadi.
"The plane was actually carrying 64 suspected mercenaries of various nationalities," Mr Mohadi said. "Further investigations also revealed that on board was military material."
State-run television broadcast footage of a white plane with the tail number N4610. Inside the aircraft, the station showed two satellite telephones, radios, blue backpacks, sleeping bags, hiking boots, an inflatable raft, paddles, bolt cutters and what appeared to be a can of mace.
No weapons were shown, but the station said officials were still going through the cargo section.
Western journalists were not shown the plane, which Mr Mohadi said had been moved to the nearby Manyame military airfield, and the government's claims could not be independently verified.
Passengers and crew were also taken to the base, where a detention barracks is located, state television reported.
But mystery still surrounds the plane, which while apparently registered with a US company was also reportedly sold to a South African company a week ago.
The intended destination of the men on board described as burly and white, and militarily equipped is unknown.
The Zimbabwean cabinet is discussing the issue and has promised more details about the plane and its cargo soon. Speculation is growing that the plane could have been heading to Equatorial Guinea, which has seen a security crackdown in recent days following reports of a coup attempt.
The plane registration has been traced to a company in Kansas.
Equatorial Guinea's information minister said 15 mercenaries had been arrested there, including several South Africans.
He described them as an advance party for the group being held in Zimbabwe.
However, another report suggests they were heading to DR Congo to guard mining operations there.
The US authorities have denied there is any connection between the plane and the government, while acknowledging it may be US-registered.
A spokesman for the Civil Aviation Authorities in South Africa, Moses Seate, has said the plane took off from the domestic Wonderboom airport, north of the capital Pretoria. If confirmed, the plane looks likely to have been attempting an illicit journey by making an international flight from a domestic airport.