Mugabe told The Herald, a government-controlled newspaper, that he would form the government and a new cabinet even without a resolution to currently stalled power-sharing talks with the MDC.
"We shall soon be setting up a government. The MDC does not want to come in apparently. This time they have been promised by the British that sanctions would be more devastating, that in six months' time the government will collapse.
"I do not know when that day will come. I wish Tsvangirai well on that day," the 84-year-old leader said.
Meanwhile, Nelson Chamisa, the MDC spokesman, said: "You can't just have a cabinet without a mandate".
Mugabe made the announcement after he officially opened the first parliament since disputed March elections.
The MDC said Mugabe had no right to open parliament.
The opposition handed a petition to the clerk of parliament, denouncing the opening of parliament as meaningless, saying it violated a deal signed in July ahead of the power-sharing talks which have been stalled for two weeks.
Edwin Mushoriwa, the spokesman for the smaller MDC faction, said: "We are not going to be part of Mugabe's government. We are actually looking forward to the conclusion of the dialogue when Mugabe and Tsvangirai form a transitional government".
A stalemate in talks between Mugabe and Tsvangirai over how to share power has undermined hopes for an agreement that might allow Zimbabwe to recover from its devastating economic decline.
The world's highest annual inflation rate of over 11mn per cent and severe food, fuel and foreign currency shortages had driven millions of Zimbabweans to neighbouring countries.