HARARE, Zimbabwe — President Robert Mugabe on Friday said veterans of his country's war for independence from Britain have threatened to go back into the bush to fight if the opposition wins the presidency.

"I'm even prepared to join the fight," the 84-year-old leader who has been in power since independence told a conference of his party's youth wing.

"We have come to a time when our independence is being questioned or being put to a test ... We are saying let us remember what we did yesterday."

Mugabe's comments come only about two weeks before he faces opposition leader Morgan Tsvangirai in a presidential runoff. The June 27 vote follows a March 29 first round in which Tsvangirai came in first in a field of four, but did not win the 50 percent plus one vote necessary to avoid a runoff.

Also Friday, a judge ordered police to bring the No. 2 opposition leader to court Saturday and explain why he should not be immediately released, an opposition lawyer said. Lawyer Selby Hwacha said the High Court order came in response to an opposition court plea.

Tendai Biti was arrested upon returning to Zimbabwe from neighboring South Africa Thursday. Police have refused to say where he was being held or when they might bring him to court. But they have said he faces a treason charge, which can carry the death penalty.

Tsvangirai, speaking on the campaign trail Friday, called the treason charge Biti faces "frivolous."

"Tendai has not committed any crime, he has not committed any offense to warrant the arrest," Tsvangirai said.

Earlier Friday, the Movement for Democratic Change said it was "deeply worried" about Biti's welfare. The party said it had dispatched a team of lawyers and human rights defenders "to every possible police station in Harare," the party said.

James McGee, the U.S. ambassador to Zimbabwe, said Biti was in a jail in western Harare, but opposition spokesman Nqobizitha Mlilo said that was not confirmed.

Also Friday, the party said Tsvangirai was released overnight after being detained by police. Tsvangirai was twice stopped by police as he tried to campaign Thursday, according to the party, held for about two hours the first time and then the second time late into the night before being released.

Such incidents have become common as Tsvangirai attempts to reach out to voters.

The Movement for Democratic Change said Friday that buses Tsvangirai was using on a campaign tour had been impounded by police, but said Tsvangirai had resumed campaigning.

In 2004, Tsvangirai was acquitted after a treason trial that lasted more than a year.

Botswana, a fellow member of the Southern African Development Community, was the first neighboring country to condemn Biti's arrest. Its Ministry of Foreign Affairs said it had summoned the Zimbabwean ambassador Thursday to express its concern over Biti's arrest and Tsvangirai's detentions.

"Botswana is alarmed by these arrests and detentions as they disrupt electoral activities of key players and intimidate the electorate thus undermining the process of holding a free, fair and democratic election," said Clifford Maribe, spokesman for the ministry in Botswana.

"We are deeply disturbed by this unfolding situation of politically motivated arrests and intolerance which pose a serious threat to an outcome that reflects the will of the people of Zimbabwe," Maribe said.

It was unusually strong language from a fellow African government. Zimbabwe's neighbors, particularly regional power South Africa, have for the most part refused to confront Mugabe.

The United States, long a sharp critic of Mugabe, said Thursday what pressure the neighbors had so far brought to bear had been ineffective. It called for immediate action by the U.N. Security Council.

In addition to being accused of orchestrating violence, Mugabe's government has in recent weeks been accused of using food as a political weapon.

The government last week ordered independent aid agencies to stop work. Mugabe has accused foreign aid agencies of working with the opposition to topple him, but the effect of the crackdown has been to make millions of hungry Zimbabweans even more dependent on his government just as they are deciding whether to keep him in power.

Aid group World Vision, which has projects across the country, appealed to the government Friday to allow delivery of basic humanitarian assistance by reversing the suspension.

"We hold grave concerns for the 1.6 million orphans and vulnerable children across the country who will now not receive critical assistance from humanitarian agencies operating in the country," Wilfred Mlay, vice president for Africa for World Vision, said in a statement.

World Vision said the suspension was keeping more than 30 local and international groups from delivering food and other aid. It said up to 4 million people were believed in need of aid.

Mugabe, in power since 1980, was lauded early in his rule for campaigning for racial reconciliation and building the economy. But in recent years, he has been accused of holding onto power through fraud and intimidation and ruining the economy.

The economic slide of what was once the region's breadbasket has been blamed on the collapse of the key agriculture sector after often violence seizures of farmland from whites. Mugabe claimed he ordered the seizures, begun in 2002, to benefit poor blacks. But many of the farms went to his loyalists.

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Associated Press Writer Sello Motseta in Gaborone, Botswana contributed to this report.