HARARE, Zimbabwe - 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 yesterday 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 yesterday.

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 yesterday.

The electoral commission notice said Justice Minister Patrick Chinamasa was 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 says 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 affecting "some of the key pillars of our structure." His party said in a statement yesterday that 33 of its supporters and activists had been killed in postelection violence.

In a statement yesterday, Amnesty International's 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."