The spokesman for Zimbabwe’s main opposition leader was assaulted by security forces as he tried to leave the country Sunday, a party official said today.

Nelson Chamisa, aide to Morgan Tsvangirai, the Movement for Democratic Change leader, was assaulted at Harare International Airport as he was leaving for Belgium via London to attend a meeting of the European Union and Africa Caribbean Pacific in Brussels, the party’s secretary general, Tendai Biti, said from Johannesburg.

“He was beaten on the head with iron bars. There was blood all over his face. He is in a critical condition at a private hospital in Harare,” Mr Biti said.

Mr Tsvangirai, who is recovering at home from injuries he receieved last week when police broke up a protest meeting, said that Mr Chamisa was in a “very critical state” and had been knocked unconscious.

The assault follows the re-arrests at the airport Saturday of three opposition activists, who were allegedly assaulted along with Mr Tsvangirai at the March 11 protest.

Meanwhile, President Robert Mugabe accused the opposition of being terrorists supported by Britain and the West, as Tsvangirai said the crisis in Zimbabwe had reached a “tipping point.”

Grace Kwinje and Sekai Holland, among the most severely injured in last week’s incident, were prevented from leaving to receive medical care, and Arthur Mutambara, leader of an opposition faction, was later also arrested at the airport.

Beatrice Mtetwa, a lawyer for Mr Mutambara, said Sunday her client was being kept at the Harare central police station, and that he was being charged with inciting public violence in relation to last week’s incident.

However, she said he and the others were never formally charged. In a letter to police, Mutambara’s lawyers called his arrest “contemptuous, arrogant and malicious defiance; of a High Court order last week that stipulated he could not be taken into custody on the same charge.

Tawanda Mutasah, director of the Open Society Initiative for Southern Africa, said the ambulance carrying Ms Kwinje and Ms Holland from Harare’s Avenues clinic to the airport, where they were to leave in a medical rescue aircraft, was stopped on the tarmac by officers from Zimbabwe’s security forces.

The women’s passports were taken and they were told they needed a clearance certificate from the Department of Health. They were then instructed to go to Harare’s central police station, but were later allowed to return to the clinic under police guard.

Zimbabwean police used tear gas, water cannon and live ammunition to crush the March 11 gathering, and beat activists, during and after arrests, according to opposition members.

Mr Mugabe, 83, has rejected the international condemnation following the arrests and alleged beating, lashing out at critics and telling them to “go hang,” and he vowed to crackdown on further protests.

Speaking at a ceremony to mark International Women’s Day in Harare on Saturday, Mr Mugabe accused the opposition party of resorting to violence sponsored by former colonial power Britain and other Western allies to oust his government, a newspaper reported Sunday.

Zimbabwe is facing a critical moment that could see the end of Mugabe’s dictatorship, Mr Tsvangirai said by telephone from Harare where he is recovering from injuries.

Photographs of his battered face were printed in newspapers around the world.

“Things are bad,” Tsvangirai told the BBC’s Sunday AM program, “but I think that this crisis has reached a tipping point, and we could see the beginning of the end of this dictatorship in whatever form.”

Tsvangirai left the hospital Friday battered but defiant, pledging to “soldier on until Zimbabwe is free.”

His supporters vowed to drive Mugabe from office with a campaign of civil disobedience.

In the interview with the BBC program, he also criticized South Africa for its role in the crisis. Calling the country a “critical player,” he said South Africa “could have been more strong,” and urged continued pressure from both the African Union and the international community, as well as individual nations such as the United States.