Mugabe said the opposition party should accept the deal signed in September or end talks over its implementation.
The MDC called for "finality" in the dialogue, saying it would not join a unity government until all its demands were met.
The power sharing deal originally signed by both parties has not come into force until today.
Though Tsvangirai was nominated to take over as the new prime minister in the new unity government, differences arose after Mugabe allocated crucial ministerial positions to his allies and not to the opposition as originally pledged.
The power sharing agreement was to end a political impasse over disputed presidential elections, which the opposition says Mugabe stole.
The failure to find an agreement in recent months has exacerbated a worsening humanitarian crisis in Zimbabwe, where more than half of the 11 million population is dependent on food aid.
A cholera epidemic has spread throught the country following the breakdown of sewage and water systems, causing the death of more that 2,200 Zimbabweans.