Mr Tsvangirai's MDC says at least 70 of its supporters have been killed
Zimbabwe's opposition MDC will announce on Monday whether the party will withdraw from the 27 June presidential run-off, a party source told the BBC.
MDC leader Morgan Tsvangirai is said to be under pressure to pull out in view of escalating poll-related violence.
The withdrawal threat may be aimed at pressuring other countries to demand a fair vote, a BBC correspondent says.
Angola's leader, one of President Robert Mugabe's closest allies, has urged him to stop the violence.
President Jose Eduardo Dos Santos advised Mr Mugabe to "observe the spirit of tolerance, respect for difference and cease all forms of intimidation and political violence".
Unless there's a change in conditions on the ground, the election will be a charade
The message was delivered through an Angolan election observer, the Angolan president's office says.
The two men were fighting colonial rule together in the 1970s and intervened together in the Democratic Republic of Congo in the 1990s.
The MDC (Movement for Democratic Change) says dozens of its supporters have been killed but President Mugabe blames the opposition for the violence.
But if the MDC does pull out, it will be handing victory to Robert Mugabe, making it a tricky decision to take, says the BBC's Caroline Hawley in Johannesburg, South Africa.
The MDC suffered at least five violent deaths of activists or their family members this week and its secretary general, Tendai Biti, was charged with treason and subversion.
In Brussels, the European Union has drafted a summit statement saying it is ready to take unspecified "additional measures against those responsible for violence".
UK Prime Minister Gordon Brown told reporters he believed President Mugabe's government was behind the violence.
"Mugabe's increasingly desperate and isolated regime has unleashed still more violence," he said.
"This is a blatant attempt to intimidate and to steal the election."
'Subjected to terror'
"Differences of opinion" have emerged among the party's senior officials over its next move, MDC spokesman Nelson Chamisa told the BBC after the leadership met in Harare.
Tendai Biti was arrested after returning from South Africa
The party, he said, needed to assess the situation in the country but if conditions did not change, the vote would be a "charade".
"We are assessing the situation as some areas are inaccessible," he added.
"People are being abducted at night. Our grass-roots activists are being subjected to terror. Some of them are staying in the bushes and mountains to avoid Zanu-PF militias.
"Unless there's a change in conditions on the ground, the election will be a charade."
The MDC says at least 70 of its supporters have been killed in recent weeks and 25,000 forced from their homes in a state-sponsored campaign of violence.
MDC rallies have also been disrupted and Mr Tsvangirai has been detained by police on several occasions.
A Harare court refused to dismiss the charges against Mr Biti, delaying his case until 7 July, more than a week after the election. However, he could still be granted bail.
The charges carry a possible death sentence. One of the accusations against him is that he portrayed President Mugabe "as an evil man".
International concern
Zimbabwe's immediate neighbours have voiced increasing concern over the validity of next week's vote, given the violence and intimidation.
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Ruling party militias hunt for opposition supporters in a Harare township, in footage filmed by US embassy staff
Tanzanian Foreign Minister Bernard Membe, head of an election monitoring team, told the BBC on Thursday that violence appeared to be "escalating throughout Zimbabwe".
The EU already has an arms embargo against Zimbabwe and has placed travel bans on and frozen the assets of President Mugabe and other senior government and ruling Zanu-PF party officials.
It urged African states to deploy as many election observers on the ground as soon as possible.
Mr Mugabe blames the sanctions for causing Zimbabwe's economic freefall.
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