The Boeing 727-100 was detained at Harare's main airport late on Sunday after its owners allegedly made "a false declaration of its cargo and crew," Home Affairs Minister Kembo Mohadi said on Monday at a news briefing.
"The plane was actually carrying 64 suspected mercenaries of various nationalities," Mohadi said. "Further investigations also revealed that on board was military material."
It was not immediately clear where the plane had come from, or what its purpose was. Mohadi gave no further information, but said full details would be released once officials have established "the true identities of the men and their ultimate mission."
Journalists were not shown the plane, which Mohadi said had been moved to a nearby military base, and the government's claims could not be independently verified.
US Embassy officials said they had not been informed of the incident and were trying to obtain details from Zimbabwe authorities.
President Robert Mugabe has repeatedly accused the United States and Britain of plotting to overthrow his autocratic regime.