Four leading Zimbabwean opposition members have been prevented from going abroad and one of them was attacked at the airport, the opposition says.
MP Nelson Chamisa said he was beaten as he tried to leave. The government denied its forces were involved.
Earlier two women activists were stopped as they tried to leave to get treatment for injuries sustained in police custody, their lawyer said.
And Arthur Mutambara was re-arrested as he was about to leave the country.
Earlier, the African Union urged Harare to respect human rights in the country.
In a statement, the pan-African body also called for a "constructive dialogue" to resolve Zimbabwe's deepening crisis.
Western criticism of Robert Mugabe's government intensified after Morgan Tsvangirai, the leader of the Movement for Democratic Change (MDC), was beaten after last Sunday's rally in Harare.
Mr Chamisa, an MDC spokesman, had been on his way to attend an Africa Caribbean Pacific-EU meeting in Belgium.
He said he had been approached by unidentified men as he got out of his car outside the departures' hall at Harare Airport.
"I was suddenly surrounded by, I think, about eight men," he said later.
"One wore a green t-shirt. The other ones had suits. Then I was hit, I think about three times... Then I fell to the ground."
Mr Chamisa said he had seen his attackers running off towards two vehicles without registration plates.
"Some women they came to me and started to help me. With a handkerchief. They were trying to stop the blood. It was really coming out," he said.
He has now been admitted to hospital in Harare where his doctor says he has a fractured skull.
"There is no security. There is no protection. All of us are at risk," Mr Chamisa said.
In an interview with the BBC's Focus on Africa programme, Zimbabwe's Minister of Information, Sikanyiso Ndlovu, denied that state security forces were involved in the attack, saying the opposition was responsible for causing havoc in Zimbabwe.
Arthur Mutambara, leader of one of the factions of the MDC, was re-arrested on Saturday, and is now being held at Harare central police station.
Grace Kwinje and Sekai Holland tried to go to South Africa to receive specialist treatment on Saturday evening, Tafadzwa Mugabe, a lawyer who accompanied them, told the BBC's World Today programme.
They were among a number of activists beaten while in police custody after being arrested last week.
Tafadzwa Mugabe said all their papers were in order but - just before boarding the flight - the authorities said the two women needed an additional "clearance letter from the ministry of health".
"This was just an arbitrary act," the lawyer said, adding that they would be taking legal action.
He said that the condition of the two women activists remained critical.
MDC leader Morgan Tsvangirai has told the BBC's Sunday AM programme that the situation has reached a critical stage.
"Well I think that this is a tipping point," he said.
"Things were bad, things are bad, but I think this crisis has reached the tipping point and we could be seeing the beginning of the end of this dictatorship."
President Mugabe has rejected Western criticism and blamed the opposition for instigating the violence.
Mr Mugabe has ruled Zimbabwe for 27 years, but there is increasing discontent over the country's economic crisis.