Tsvangirai's motion in the High Court seeks to overturn Mugabe's naming of provincial governors without his consent, The Wall Street Journal reported. The prime minister said that move violated the governing agreement.
A "prime minister seeking to reverse constitutional appointments by the president is ridiculous. But we will respond fully in court," said Rugare Gumbo, spokesman for Mugabe's Zanu PF party.
South African President Jacob Zuma, the official mediator in the Zimbabwe confrontation, was due to visit Friday in a bid to draft a plan for elections that would resolve the conflict.
The last election in 2008 was tainted by widespread violence, mainly directed at Tsvangirai's supporters. After Tsvangirai pulled out of a presidential runoff to avoid more bloodshed, mediators brought the rivals into the power-sharing deal.
Zanu PF says it wants elections by June and Mugabe is almost certain to be its nominee despite his age, 86, and reports of poor health.
Tsvangirai's Movement for Democratic Change is calling for reforms to guarantee an honest vote first.
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