Silvanos Mudzova, a member of the pressure group Tajamuka, said he had been abducted from his home in Harare on Tuesday night by unknown men.
He told the BBC he had been blindfolded and tortured with sharp objects.
He was also electrocuted on several parts of his body, including his genitals, Mr Mudzova said.
He said his captors had been armed.
The activist said his attackers had interrogated him, wanting to know who was funding his Tajamuka pressure group and why they were campaigning against President Mugabe's government.
The spokesman for the group was also arrested but granted bail on Wednesday.
Despite the latest government ban on protests in the capital, Harare, a massive demonstration has been planned for Saturday, according to Mr Mudzova.
Zimbabwe has seen a wave of protests recently over the declining economy.
Earlier this month, President Mugabe criticised judges who gave permission for anti-government protests which later turned violent.
He said the judges had shown a reckless disregard for peace and warned they should take care when making decisions.
The opposition has accused Mr Mugabe of trying to intimidate the judiciary.
Last week, Zimbabwe's high court overturned a two-week ban by police on demonstrations in Harare.
Activists who are opposed to President Robert Mugabe and his government challenged the ban in court.
The president recently warned protesters there would be no Zimbabwean uprising similar to the "Arab Spring".

The bank has previously said that a "significant percent" of its London workforce would be moved if Brexit became a reality.
The bank employs around 5,000 staff at its offices in London.
Earlier, the Lloyds of London insurance market said that after Brexit some of its business would move to the EU.
"We currently employ more than 5,000 people in London, and probably 20% to 30% of our workforce could be affected," the UBS chief executive, Sergio Ermotti, told the Japanese business newspaper Nikkei.
"We believe that London will continue to be an important financial centre, although maybe not as important as it is today," he added.
The UK voted narrowly to leave the EU in the Brexit referendum in June.
Since then the government and many UK businesses have been grappling both with the process of negotiating the divorce - which has not yet been formally triggered - and the eventual economic, social and political consequences of such a move.
Mr Ermotti said that it was too early for the bank to take any decision about moving its operations from London, but it was "well prepared" for any outcome of the impending Brexit negotiations.

Notices of intended prosecution claimed 991 drivers had broken a 40mph (64km/h) speed limit in Conwy tunnel last October.
But the limit had been imposed for night maintenance only and not lifted in the morning as it should have been.
Within days the drivers got an apology in the post.
North Wales Police released the figures in a Freedom of Information reply.
The force said: "The issue was caused by an administration error surrounding the enforcement period.
"North Wales police do not record the cost of cancelling notices."
A man who has been at the forefront of protests against Zimbabwean President Robert Mugabe is in hospital after allegedly being beaten and tortured.
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The Swiss banking group UBS says that up to 1,500 of its jobs in London may be moved abroad once the UK leaves the European Union.
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Nearly 1,000 drivers were wrongly sent speeding notices after a temporary limit on a north Wales road was not lifted, figures have shown.