Human rights activists and monitors say there is growing evidence of police brutality against people who have been detained in Belarus as protests over the disputed election continue.
Some of the 6,700 people detained since Sunday have been released and allege mistreatment, including beatings.
Amnesty International said the accounts suggested "widespread torture".
Thursday marked the fifth day of protests amid widespread allegations of vote rigging.
Sunday's vote saw long-term President Alexander Lukashenko elected for another term.
In a statement on state television, Senate Speaker Natalya Kochanova said the president had ordered an investigation into the mass detention of protesters, and that more than 1,000 had been freed.
Some of the released prisoners shared pictures on messaging app Nexta show their bruised and swollen bodies, including injuries to backs and buttocks, that were allegedly caused by police.
Amnesty International said people who had been arrested described being stripped naked, beaten and threatened with rape.
"Former detainees told us that detention centres have become torture chambers, where protesters are forced to lie in the dirt while police kick and beat them with truncheons," said Marie Struthers, Amnesty International's Director for Eastern Europe and Central Asia.
In audio shared by a BBC journalist, screams could be heard from inside Okrestina detention centre in the capital Minsk.
Separately, a group of five UN human rights experts said the response of the security forces to peaceful protests had been harsh, with frequent use of excessive, unnecessary and indiscriminate force, AFP news agency reports.
"Authorities only seem interested in quickly dispersing the protests and arresting as many people as possible," they said in a joint statement.
Interior Minister Yuri Karayev said he took responsibility for people being injured during the protests, and that he would like to apologise to those who got caught up in the violence.
Observers say the official statements and the release of the prisoners suggested a more conciliatory approach following public outrage and international condemnation at the police response. EU foreign ministers were due to consider imposing sanctions on Belarus.
Officials have confirmed two deaths during the unrest. One demonstrator died during a protest in Minsk on Monday while a man died after being arrested in the south-eastern city of Gomel on Wednesday.
Shock at police brutality as testimonies mount
By Olga Ivshina, BBC Russian
The body of evidence of police brutality, both in the streets and inside remand prisons, is mounting. Detainees include not only opposition activists, but also many journalists and passers-by.
One of the released journalists, Nikita Telizhenko of the Russian Znak.com news website, published a harrowing account of three days inside prison. Now back in Russia, he describes people lying on the floor of a detention centre, piled on top of each other, in a pool of blood and excrement. They were not allowed to use the toilet for hours on end or even change position.
He says he saw seriously injured people, with broken limbs and severe bruising, not only left without medical help, but kicked and beaten more by the guards.
Telizhenko's testimony is confirmed by countless posts on social media – photos, videos, stories. I spoke to an American woman who was visiting her Belarusian boyfriend in Minsk – he got detained for no apparent reason. Not only had he not been protesting, but he was asleep in bed when the police came to his flat, kicked down the door and took him away.
More about the protests in Belarus
What else happened on Thursday?
Footage broadcast by messaging app Nexta on Thursday night appeared to show protests continuing in the streets with demonstrators chanting "get out".
Tearful relatives gathered outside a jail north of the capital Minsk, hoping to be reunited with their loved ones or obtain information on their whereabouts.
Several strikes have been reported at state-owned factories, where workers object to the violent treatment of protesters. Hundreds of employees were seen walking out at truck-maker Belaz in Zhodino to the north-east of the capital.