Zimbabwe police say four opposition officials were prevented from going abroad at the weekend as they may still be charged with inciting violence.
Sekai Holland wanted to travel for medical treatment
Two women officials wanted to go to South Africa for specialist treatment, after they said they had been beaten in police custody.
Scores of activists were arrested and allegedly assaulted after police broke up an opposition meeting last weekend.
An opposition MP said he was attacked by unknown men outside Harare airport.
Those arrested were not charged when they appeared in court last week and have since sought medical treatment for their injuries.
"The case is still pending and they have to appear in court first," police spokesman Wayne Bvudzijena told Reuters news agency.
"I understand the dockets are ready so the case should proceed any time."
The police accuse the opposition Movement for Democratic Change of starting the violence, which it strongly denies.
Meanwhile, the AP news agency reports that the US ambassador walked out of a meeting with Zimbabwe's foreign minister, at which he warned foreign diplomats against backing the MDC or risk being expelled from the country.
The United States has said it holds Zimbabwe's President Robert Mugabe "personally responsible" for the recent attacks on opposition figures.
President Mugabe has said his Western critics can "go hang".
He has long accused the West of trying to oust him and a minister accused the West of double standards.
"We find it disturbing in that we are not hearing anything on the three women police officers who are struggling for their lives in hospital and about three police officers who were badly beaten up by the MDC," junior Information Minister Bright Matonga told the AFP news agency.
The police say the MDC "armed wing" firebombed police camps last week.
MP Nelson Chamisa said he was severely beaten on Sunday as he tried to leave the country.
Senior MDC officials Grace Kwinje and Sekai Holland attempted to go to South Africa to receive medical 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 who say they were beaten while in police custody after being arrested last week.
Arthur Mutambara, the leader of one of the factions of the MDC, was re-arrested on Saturday, as he tried to leave the country.
The travel ban would also apply to Morgan Tsvangirai, leader of the other MDC faction, who needed stitches in a head wound.
Mr Tsvangirai told the BBC's Sunday AM programme that the situation had reached a critical stage.
"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," he said.