The NGO says it has photos, video recordings and testimony that can be used as evidence of rights violations. The UN also accused Belarus of torturing detainees, a charge the government denies.
Belarusian riot police block the road to stop demonstrators during an opposition rally to protest the official presidential election results in Minsk
Detained protesters in Belarus have been systematically tortured, human rights group Amnesty International said in a report released on Wednesday.
Belarusians have been taking to the streets against President Alexander Lukashenko, once dubbed Europe's last dictator, since his disputed re-election in August.
The Amnesty report cites testimonies from detainees who say they were stripped naked, beaten and deprived of food, drinking water or medical care for days.
Amnesty says it has photos, video recordings and testimony of detainees, victims and witnesses that can be used as evidence of rights violations.
But Marie Struthers, the group's director for eastern Europe and central Asia, said the country's justice system "not only protects police with anonymity, but also encourages intimidation and further violence against victims and witnesses."
She said an international investigation was needed into "the unprecedented scale of the ongoing human rights violations" and the perpetrators must be prosecuted.
Last month, UN human rights chief Michelle Bachelet urged the government to release all people unlawfully arrested in post-election protests and investigate some 2,000 complaints of torture or ill-treatment in custody.
The Belarusian government has repeatedly all of the allegations.
Lukashenko, who has ruled Belarus for 26 years, claimed victory in the elections with 80.1% of the vote.