Mugabe supporters insists they will not accept an MDC victory
Hundreds of supporters of Zimbabwe's President Robert Mugabe have occupied a stadium in the capital, Harare, where the opposition had planned a key rally.
Zanu-PF supporters, armed with clubs and stones, beat bystanders outside Glamis stadium, witnesses say.
A spokesman for the opposition Movement for Democratic Change said it planned to go ahead with the rally. MDC leader Morgan Tsvangirai was due to attend.
The MDC says it will decide by Monday on whether to contest run-off vote.
It says at least 70 of its supporters have been killed in the run-up to the 27 June vote, which will see Mr Tsvangirai challenge Mr Mugabe.
The MDC is due to meet on Sunday to discuss its possible withdrawal from the vote.
"We have to evaluate whether it is worth spilling blood in the name of an election" MDC spokesman Nelson Chamisa told Reuters news agency.
Sunday's rally was due to go ahead after the High Court on Saturday overturned a police ban. According to eyewitnesses, hundreds of Zanu-PF youth militia have thronged the streets around the Glamis stadium, while hundreds more are inside.
Carrying clubs, sticks and stones, they have been stopping cars, demanding people sing party slogans and in some cases show party cards.
Witnesses said they have been beating people outside the stadium.
Uniformed riot police - and observers from the South African Development Community - are on the scene. A Zimbabwean journalist has seen riot police fire tear gas, forcing some Zanu-PF supporters to run away.
The opposition rally had been due to begin around noon local time (1000 GMT) - and the party says it is determined to go ahead.
Morgan Tsvangirai say he won the vote outright in March
However a party spokesman told the BBC that given the large numbers of armed Zanu-PF supporters in and around the stadium, violence is likely.
On Saturday, South Africa sent two mediators to Harare, just days after its President, Thabo Mbeki, went to Zimbabwe himself, for separate talks with Mr Mugabe and Mr Tsvangirai.
The BBC's Peter Biles, in Johannesburg, says this is possibly a last ditch effort to persuade Mr Mugabe to cancel the election run-off, and to persuade both sides to begin negotiations on a government of national unity.
It is widely accepted that in the present circumstances, with serious political violence, the election will not resolve Zimbabwe's problems, our correspondent adds.
The MDC says its members have been beaten, and its supporters evicted from their homes, forcing it to campaign in near secrecy.
The party's secretary general, Tendai Biti, is under arrest charged with treason.
Mr Mugabe has accused the MDC of acting in the interest of Britain, the former colonial power, and other Western countries.
Zimbabwe's other immediate neighbours have also added their voices to increasing international concern over the validity of the run-off.
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