Police armed with batons, guns and tear gas, supported by others in armoured trucks, arrived at the stadium and arrested dozens of supporters arriving for the rally, witnesses said.
Earlier, police had tried to ban the event, saying they had not been given enough notice about it.
But the Zimbabwe High Court said on Saturday that the government should allow the MDC rally to go ahead regardless.
Detained by the police
Elias Mudzuri, the MDC's national organising secretary, said about 200 young people who had been on security duty at the sports ground overnight had been attacked and either driven away or detained by the police.
But he said the party was still hoping to hold the rally, where Morgan Tsvangirai, the MDC leader was due to speak.
"We are not cancelling the rally," he said.
"What they [the police] are doing is in defiance of the court order, and among those arrested is a provincial member who had gone to serve the police authorities with the court order."
The MDC had planned to use the rally, in the capital's Highfield shanty town area, to launch its presidential election campaign.
The election is due in March 2008 but the ruling ZANU-PF party, headed by Robert Mugabe, Zimbabwe's president, plans to put off the elections until 2010.
Growing isolation
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"Zimbabwe is not gaining friends and those friends it had are becoming lukewarm"
Mosiuoa Lekota, South Africa's defence minister |
Political tension is rising in Zimbabwe, where food shortages, inflation at nearly 1,600 per cent and unemployment above 80 per cent have caused great hardship and poverty.
Last week, Mugabe was left off a list of African leaders invited by Jacques Chirac, the French president, to attend the France-Africa summit in Cannes.
Four years previously the French ignored an EU travel ban on Mugabe, inviting him to attend the conference.
The decision not to invite Mugabe to this year's conference has highlighted the isolation of a leader whose last state visit was to Iran in November.
Eldred Masungure, a professor of political science at the University of Zimbabwe, said: "Zimbabwe is not gaining friends and those friends it had are becoming lukewarm."
Last week Mosiuoa Lekota, South Africa's defence minister, called Zimbabwe "a problem" that needed to be addressed by the whole of the region.