The government of President Robert Mugabe, who will face Tsvangirai in a run-off election on June 27, said it was unaware of any plot and the allegation was a media stunt.

Tsvangirai postponed his return from Europe on Saturday after his Movement for Democratic Change said it had discovered the plot. He had spent more than a month abroad.

"We know there are 18 snipers, and the military intelligence directorate is in charge of this," Tendai Biti, MDC secretary general, told Reuters before a news conference in Nairobi.

Biti said Tsvangirai would still return "very soon."

Deputy Information Minister Bright Matonga told Reuters: "Like I've said before, we know he did not leave the country on security grounds. We are not aware of any plot against anyone, but we're sure he's playing to the international media gallery."

According to official results, which did not emerge for five weeks after the March 29 election, Tsvangirai beat Mugabe but fell short of the absolute majority needed for outright victory.

RECOVERY?

Zimbabweans hope the runoff will start the process of recovery from an economic collapse that has brought 165,000 percent inflation, 80 percent unemployment, chronic food and fuel shortages and sent thousands fleeing to nearby countries.

On Sunday, the MDC said it would "bury" Mugabe in the runoff, ending his uninterrupted rule since independence from Britain in 1980.

The MDC says Tsvangirai won the first vote outright and accuses Mugabe's ZANU-PF of both electoral fraud and of unleashing its militias against opposition supporters in the countryside to ensure a second round victory.

The MDC says at least 40 of its supporters have been killed.

In a parallel vote on March 29, ZANU-PF lost control of parliament for the first time since the 84-year-old Mugabe took power. The veteran leader told the party the result was "disastrous" and vowed not to go down to an opposition he said was backed by Western imperialists.

Senegalese President Abdoulaye Wade said at the weekend that the African Union should oversee the second round.

The majority of the 3 million economic refugees from Zimbabwe have fled to South Africa, where they are now among the victims of a wave of xenophobic violence in poor townships that killed at least 13 people over the weekend.