Prime Minister Robert Mugabe unexpectedly called off a scheduled press conference today amid rumors that two of his senior ministers would be demoted in a long-awaited Cabinet shake-up.

Reports of changes in the nine-month-old government involve Joshua Nkomo, Mugabe's political rival who was leader of a guerrilla faction during the Rhodesian civil war, and Edgar Tekere, the secretary general of Mugabe's party who was involved in the shooting death of a white farmer recently.

According to these reports, Tekere will lose his position as minister of manpower planning, and Nkomo, whose party is nominally a coalition partner in the government in which he has held the post of home affairs minister, will be demoted to minister of public service.

The abrupt postponement of the press conference at which Mugabe planned to announce the first changes in his Cabinet suggested continued political difficulties.

A government spokesman said Mugabe was still consulting with his Cabinet ministers over the changes late today, and his press conference was rescheduled for Saturday. The reshuffle has been in the works for about two months, and the prime minister said in an interview 11 days ago that it would be announced "very soon."

Officials here refused all comment, but government sensitivity was evident in an incident last night in which a journalist who reported that changes were impending was threatened and held informally for questioning for about six hours by Rex Nhongo, commander of the guerrilla forces that helped bring Mugabe to power.

The incident came at a particularly delicate time since just last week the Ministry of Information announced the takeover of the country's major privately owned newspapers by a government-funded press trust.