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".
A man who has been at the forefront of protests against Zimbabwean President Robert Mugabe is in hospital after allegedly being beaten and tortured.