Mugabe used temporary presidential powers to set the date for a vote that would put an end to their uneasy power-sharing government.
"Today, early in the morning, I received a letter from president Mugabe (proclaiming)... July 31 as the election day," Tsvangirai said.
"President Mugabe is acting unlawfully and unconstitutionally," said Tsvangirai. "As prime minister I cannot and will not accept this."
Tsvangirai vowed to take the matter to court and urged regional leaders to step in and stop Mugabe.
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Tsvangirai has vowed to veto any election date that comes before democratic reforms are put into place, fearing Mugabe's ZANU-PF party may once again attempt to manipulate the vote.
In 2008 Tsvangirai led Mugabe in the first round of the presidential ballot, but withdrew from the run off amid violence that left scores of supporters dead and thousands injured.
"I am not a character who is going to commit suicide," Tsvangirai said.
Tsvangirai called on regional leaders of the Southern African Development Community (SADC) -- which pushed the pair into a unity government after the violence of 2008 -- to act when they meet in Maputo on Saturday.
He described Mugabe's move as "an unmitigated frontal and rear attack" on the 15-member regional grouping.
Under the power-sharing deal Mugabe must consult Tsvangirai's Movement for Democratic Change (MDC) about the date of the presidential, parliamentary and local elections.
"The interpretation of the word 'consultation' is a bit of a challenge," said Trevor Maisiri of the International Crisis Group.
"If you speak to ZANU-PF they'll say consultation means the president can ask the principals what they think. With their suggestions he can make a decision."
"The MDC say 'we'll make the decision together'."
Given the constitutional court deadline he said "it became inexpedient to await the passage through parliament of the electoral amendment bill".