HARARE, Zimbabwe (CNN) -- Zimbabwe's opposition party said four of its activists and the wife of Harare's mayor -- an opposition member -- have been killed by supporters of President Robert Mugabe, just days ahead of next week's presidential runoff.

President Robert Mugabe says war veterans will take up arms again if Tsvangirai wins.

Thursday's report came as U.S. Secretary of State Condoleezza Rice accused Mugabe of sponsoring efforts to starve, beat and kill supporters of his opponent Morgan Tzvangirai so he can win the election.

Rice was speaking on the same day that Tendai Biti, the opposition Movement for Democratic Change's secretary-general, was charged with treason after being held in jail without charges for a week. If convicted, he could face the death penalty.

His arrest and treason charges have been criticized by African and international leaders who characterize it as a ploy by Mugabe supporters to intimidate the opposition party before his June 27 runoff against Tsvangirai.

A spokesman for the MDC blamed Mugabe's Zanu-PF party for the five most recent deaths, saying they brought to 70 the number of MDC party members killed since a bitterly contested election three months ago.

The body of the mayor's wife, 27-year-old Abigail Chiroto, was found in a mortuary close to the couple's house north of Harare. She had been beaten so severely with rocks and iron bars that her face was almost unrecognizable, MDC spokesman Nelson Chamisa said. Watch CNN's Nkepile Mabuse report on the violience »

Chiroto was kidnapped, along with her 4-year-old son, on Tuesday. Some of her kidnappers wore military uniforms, Chamisa said. Chiroto's son was released unharmed.

Her husband, Emmanuel Chiroto, is an MDC member who was recently elected mayor of Harare. He was not home at the time of the kidnapping.

Also Thursday, the MDC said that four other activists were found dead in Chitungwiza. The victims' bodies showed evidence that "they were heavily tortured until they died," an MDC statement said.

"It's unbelievable the way people are being killed or murdered," Chiroto said. "It's almost mass butchering."

Police confirmed the deaths of the activists but did not link the victims to any political party.

Mugabe's party denied any part in the deaths, saying MDC officials made such accusations frequently.

"They are claiming anyone who dies. They phone CNN," said Bright Matonga, a Zanu-PF spokesman. "Whenever someone dies in the hospital, they rush to claim them."

Matonga said Harare was run by a commission. "There is no MDC mayor in Zimbabwe," he said. "There is no newly elected mayor in Harare." Learn more about Zimbabwe »

In New York, Rice convened a meeting about the situation in Zimbabwe at the United Nations on Thursday.

"Mugabe is increasing violence against [the] opposition. ... President Mugabe has squandered the promise of the very nation that was hailed as the jewel of Africa," Rice said at a roundtable discussion attended by representatives from many international governments.

"Clearly we have reached a point where broader, stronger, international effort is needed," she added.

On Friday, a magistrate judge will determine whether prosecutors have enough evidence to proceed with a trial against Biti. The judge also will determine whether Biti will be granted bail or will continue to be jailed until the trial.

Biti was charged Thursday with treason, communicating false information prejudicial to the state, bringing the office of the president into disrepute and causing disaffection among armed forces, according to a journalist who was in the Harare courtroom. He denies all the charges. Watch Biti arrive for his court hearing »

Biti, wearing a red jacket and looking exhausted, held his head in his hands as the proceedings took place in the packed courtroom.

"I think that one must express very serious objection to the manner in which this whole case is being handled," said Tsvangirai, who was in court. "The accusations are frivolous."

Three of the charges stem from a document titled "The Transitional Strategy," which Biti is said to have written ahead of the March 29 election. Biti is denying that he wrote the document, and his lawyers described it as "doctored."

The fourth charge of communicating falsehoods alleges that Biti announced that Tsvangirai had won the March presidential election by an outright majority, meaning no runoff vote would be needed.

In recent weeks, opposition groups and churches have reported numerous cases of kidnappings, torture and other violence in the country targeting opponents of Mugabe. Zanu-PF members have been suspected of being behind the acts.

Zanu-PF claims that the MDC is behind the violence. It said MDC members attacked the mayor of Kadoma, a city 140 km (87 miles) southwest of Harare. It also accused MDC activists of causing millions of dollars damage to private business.

Last week, Mugabe warned that veterans he commanded in his country's liberation war nearly three decades ago would take up arms again if Tsvangirai won. Watch an ad vilifying Tsvangirai »

The MDC is using word of mouth and file-sharing sites such as YouTube to disseminate its campaign advertisements, claiming that they are banned by state media.

The group sent advertisements to supporters via e-mail, asking them to forward them to their friends and relatives.

"Since the regime has denied the MDC access to state media, please send the adverts to as many people as you can," the e-mail says. "A new Zimbabwe is near. The dictator is finished. Let's finish it!"

Mugabe has been Zimbabwe's only leader since the war ended in 1980 but is blamed for the economic collapse of a country once considered a regional breadbasket.

Zimbabweans increasingly are unable to afford food and other essentials, with agriculture paralyzed by land reform and the world's highest rate of inflation.

Police have arrested Tsvangirai several times in the weeks leading up to the runoff, most recently on Saturday, with 11 other officials and supporters from his party.

South African President Thabo Mbeki met Mugabe and Tsvangirai on Wednesday in the hope of quelling tensions.

Mbeki was visiting in his capacity as mediator with the 14-nation Southern African Development Community, the South African government said Wednesday.

He is under domestic and international pressure for his perceived conciliatory stance toward Mugabe, but the South African leader recently said he would continue his quiet diplomacy despite recent events.

CNN's Nkepile Mabuse contributed to this report.

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