Tsvangirai beat Mugabe in the March 29 presidential poll but failed to win an absolute majority. His Movement for Democractic Change, which has accused Mugabe's government of cheating in past elections, fears it will rig the results of the June 27 run-off.

"If the president loses, he will be the first one to go on national television to acknowledge the result to the people," Emmerson Mnangagwa, a government minister and Mugabe's chief election agent, told the newspaper.

But Mnangagwa added that Mugabe and his ruling ZANU-PF party were confident they would win the second round of voting.

The 84-year-old Zimbabwean ruler kicked off his re-election campaign on Sunday, accusing the United States of political interference in Zimbabwe's affairs and the MDC of training youths to engage in political violence.

He threatened to kick out U.S. ambassador James McGee and said the State Department's top Africa envoy had behaved like a prostitute for suggesting that Tsvangirai had won the March 29 poll. The MDC won control of parliament in a parallel election.

Mugabe, who has ruled Zimbabwe since independence from Britain in 1980, routinely accuses the United States and Britain of backing the MDC to punish him for seizing thousands of white-owned farms since 2000.

He also says Western sabotage is to blame for Zimbabwe's economic meltdown. The southern African nation is struggling with inflation of more than 165,000 percent, unemployment of 80 percent and chronic food and fuel shortages.

Some 3.5 million people have fled to South Africa and other countries to escape poverty and malnutrition.

Mugabe and his ZANU-PF party have signalled they intend to pursue a grassroots campaign focused on voter canvassing and small village meetings ahead of the run-off, eschewing the mass rallies they have favoured since 1980.

The MDC draws much of its backing from the capital Harare and other cities. Mugabe, whose popularity has plunged as the economy has collapsed, needs to be able to offset that if he is going to win.

The Zimbabwean ruler is still admired by many of his own citizens and others in Africa for leading the battle to end British colonial rule, and a grassroots campaign may tap into that legacy.

Tsvangirai also launched his campaign after returning to Zimbabwe on Saturday for the first time since early April, repeating his demand that the government end the political violence that has engulfed the nation since the March polls.

The MDC says dozens of its supporters have been killed or beaten in an intimidation campaign orchestrated by ZANU-PF. The ruling party says the MDC is responsible for the bloodshed.