An MDC official said that scores of its sympathisers and activists had been beaten late Wednesday in the Harare suburb of Highfield.
In Bulawayo, around 40 activists were arrested, the party said, although police later put the figure at 27.
The claim by the opposition party came on the fourth day of mass anti-government action, including mass job stayaways and democracy marches.
In private press advertisements the MDC urged people to defy government warnings and take to the streets in their millions on Friday for what it promised would be the "D-Day" in its week-long protests.
"This is the moment you have been waiting for. Tomorrow, Friday June 6, 2003, is D-Day," the MDC said on the penultimate day of protests, dubbed the "final push".
In a full-page advertisement it called on its supporters not to be intimidated.
"Don't be afraid. No force is stronger than you. Victory is in sight".
Pearson Mungofa, the MDC representative for Highfield, said supporters of the ruling Zimbabwe African National Union - Patriotic Front (ZANU-PF) wearing army uniforms carried out the assaults on Wednesday night.
Those assaulted included opposition party activists and "those who they think sympathise with the MDC," Mungofa said.
Meanwhile a police statement said that ten suspected MDC activists had been arrested in the Harare suburb of Budiriro "for organising and inciting public violence".
No details were given.
One person was seriously injured in the western resort town of Victoria Falls after a bus was stoned late Wednesday night in a "politically motivated attack", police spokesperson Wayne Bvudzijena added.
The MDC had called the anti-government protests to oust Mugabe from power or at least force him to discuss the country's economic, political and social problems with the opposition.
But street marches failed to take off Monday as government security forces descended on the would-be protesters using teargas, dogs, batons and even live ammunition.
Yesterday's fresh call to action came as most businesses remained closed, with less than half the shops open in central Harare.
But police and the army maintained a heavy presence. Military helicopters hovered over the capital for most of the morning.
Mugabe has defended the use of force by his security forces, saying they had acted in the interest of peace and stability.
"It is sad when we are forced as a government to use teargas against our own youth who are being misled, but we have to do it in the interests of peace and security," Mugabe told South Africa's SABC television news.
In the second city of Bulawayo, a news correspondent estimated that around one shop in four and almost all banks were open yesterday.
Many banks were reportedly forced to open by the police although some of them did not have any cash to give to clients.
The High Court last weekend declared the MDC mass action against Mugabe's government illegal and warned that demonstrators would face "the full wrath of the law" if they defied the ban on the protests.
Meanwhile, civil rights groups have condemned the reports of ongoing violence.
In a statement the Crisis in Zimbabwe Coalition (CZC) said it was "disgusted with the level of harassment, intimidation and brutality" meted out against Zimbabweans.
The Zimbabwe Association of Doctors for Human Rights (ZADHR) also issued a statement condemning the violence. – Sapa-AFP.
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