HARARE, Zimbabwe — Zimbabwe's president officially announced Thursday that elections will go ahead next month despite opposition from the prime minister, setting the stage for a political standoff between the longtime rivals.

In an official government notice Thursday, President Robert Mugabe said that he was empowered to set the presidential election for July 31.

Legislative and local elections will also take place that day, and a presidential runoff will be held Sept. 11 if necessary, the notice said.

Prime Minister Morgan Tsvangirai has strongly objected to holding elections next month, accusing Mugabe of choosing the date unilaterally and saying that he will not accept Mugabe's decision.

"I appeal to fellow Zimbabweans to remain calm and vigilant in the face of this provocation and illegality," Tsvangirai said earlier Thursday before the notice was officially released. He said he had received a letter from Mugabe informing him of the decision.

Mugabe was forced by regional leaders to form a coalition government with former opposition leader Tsvangirai after violent and disputed elections in 2008. The coalition agreement demands its leaders consult and agree on major policy decisions including a timeframe for fresh polls.

The longtime president has said he is abiding by a court ruling calling for the vote to go ahead in July.

Tsvangirai, though, claims the lawsuit was instigated by Mugabe's loyalists eager for early polls so that they can take advantage of loopholes in the electoral laws to rig the vote.

Tsvangirai said Thursday that Mugabe is acting "unconstitutionally and unlawfully" because the special powers that he is claiming are not enshrined in the nation's new constitution, which was overwhelmingly accepted in a referendum vote in March.

Tsvangirai also said Mugabe's push for early polls is a deliberate attempt to avoid ending bias in his dominant, loyalist state media and carry out reforms in the police and military blamed for state-orchestrated violence in previous elections.

"I will not let Zimbabweans be frog-marched into another illegitimate and violent election," Tsvangirai said Thursday.

Mugabe has repeatedly dismissed calls for reforms in the security services.

Senior generals have vowed their allegiance to Mugabe and have refused to salute Tsvangirai since he became prime minister in 2009, arguing he did not take part in the guerilla war that ended colonial rule and swept Mugabe to power in 1980.

In an official government notice published earlier Thursday, Mugabe said he has amended several electoral laws and announced June 27 as the end of a 30-day registration period that began on Monday. Under the new constitution, 30 days are required for final voter registration.

Veritas, an independent legal research group, said Thursday that Mugabe's amended election laws still have to be passed by the Harare parliament.

"Presidential powers cannot be used to do by regulation what the constitution says must be provided for by an act of parliament," the group said.