HARARE (Sapa-AP) - The presidential runoff between President Robert Mugabe and opposition leader Morgan Tsvangirai will be held by July 31, election officials said - but the opposition insisted Thursday that it should be next week.

An official government notice issued late Wednesday extended the deadline for holding the runoff to 90 days - beyond the legally required 21 days -after the release of election results, The Herald newspaper, a government mouthpiece, reported Thursday.

Tendai Biti, secretary-general for Tsvangirai's Movement for Democratic Change, called that decision "irresponsible."

"This country cannot afford 90 days" of more violence and instability and deteriorating economic conditions, he said Thursday.

The electoral commission notice said Justice Minister Patrick Chinamasa is empowered under election laws to extend the original 21-day period for a runoff to 90 days. The original 21 days would end May 24. The opposition has called for a runoff on May 23.

Tsvangirai claims he won the March 29 presidential race outright, beating Mugabe and two other candidates. But official results released May 2, more than a month after the poll, show he did not win enough votes to avoid a second round against Mugabe.

The opposition and local and international human rights groups have accused Mugabe's party of using delays to mount a campaign of violence and intimidation against opposition supporters.

Biti, speaking to reporters in Johannesburg, South Africa, said violence was intensifying and now affecting "some of the key pillars of our structure." His party said in a statement Thursday that 33 of its supporters and activists had been killed in postelection violence.

In a statement Thursday, Amnesty International Zimbabwe researcher Simeon Mawanza expressed particular concern about people in remote rural areas.

"The situation for these victims of violence is dire," Mawanza said.

"Humanitarian organizations and local non-governmental organizations are being targeted for helping victims, who are being blocked from receiving medical assistance."

Biti called on the Southern African Development Community to hold an emergency summit to address the opposition's call for a runoff by May 23 and for the regional organization to guarantee security, fairness and freedom of the vote.

He said the opposition remained determined to participate in the runoff. He also said he and Tsvangirai, who have been out of the country since shortly after the March 29 election, would soon be returning to Zimbabwe.

Biti noted that this weekend, his party planned a campaign rally and a caucus of members elected to parliament, indicating Tsvangirai would be in Zimbabwe for those events. He said he would return some time after Tsvangirai.

Tsvangirai's party won control of parliament in legislative elections held alongside the presidential vote. It was the first time since independence that Mugabe's party lost control of parliament.

The U.S. State Department reiterated its call for a free and fair vote. "It will be up to us, as well as, in particular, friends and neighbors of Zimbabwe in the region, to keep the pressure on the Zimbabwean electoral commission and officials in Zimbabwe to create the atmosphere that will allow for a free and fair runoff," spokesman Sean McCormack said in Washington.

Meanwhile, Zimbabwe's central bank unveiled a new half-billion Zimbabwean dollar bank note Thursday.

The new bill and three others for 5 billion, 25 billion and 50 billion Zimbabwe dollars, called "special agro" checks intended for purchases and sales involved in farm production, were going into circulation next Tuesday, the central bank said.

Earlier this month, the bank floated the local currency exchange rate through commercial banks, where a single U.S. dollar sold Wednesday for around 240 million Zimbabwe dollars, slightly higher than the dominant black market rate for hard currency.

That change saw prices of goods soar, with unofficial estimates putting annual inflation at more than 700,000 percent.

Official inflation was given in February at 165,000 percent, and no further official figures have been released.

"Prices are now doubling every week instead of every month, and it is hard to see how we can survive to the end of June or how an election will be feasible at all if things continue to deteriorate at this pace," Harare economist John Robertson said.

The central bank said the "agro" checks, similar in appearance to the nation's existing range of bills, will be accepted by retailers and banks up to the end of the year.

The previous highest denomination bill was for 250 million Zimbabwe dollars (US$1; €0.64), enough to buy about two loaves of bread.