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”.
Belarus has seen five days of protests amid widespread allegations of vote rigging.
After Sunday’s vote, President Alexander Lukashenko was declared the victor by election authorities, although supporters of the main opposition figure, Svetlana Tikhanovskaya, insist she won.
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 showing their bruised and swollen bodies, including injuries to backs and buttocks, that were allegedly caused by police.
“They beat people ferociously, with impunity, and they arrest anyone. We were forced to stand in the yard all night. We could hear women being beaten. I don’t understand such cruelty,” one man said as he showed the BBC his bruising.
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 the 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.
Workers in Minsk and Grodno in the west also organised strikes and walkouts. Women in their thousands carrying flowers formed “solidarity chains” in Minsk and other cities to protest police brutality.
Video footage shared on social media showed opposition figure Maria Kolesnikova joining the female protesters in Minsk, holding a bunch of flowers. She was one of three women who pooled their resources to spearhead the opposition. The other two have left the country.
Veronika Tsepkalo fled Belarus on the day of the vote while the main opposition candidate in the election, Svetlana Tikhanovskaya, was briefly detained on Monday before being forced to leave for Lithuania.
Ms Tikhanovskaya, 37, released a video saying she made the “very difficult decision” to leave because of her children.
The opposition candidate was a stay-at-home mother until she entered the race after her husband was arrested and blocked from registering for the vote.
She became Mr Lukashenko’s toughest opposition challenge in years, leading large rallies in the lead up to the vote.
But Mr Lukashenko dismissed her bid, saying a woman could not lead Belarus. Aged 65, he has ruled the former Soviet country since 1994 and has described opposition supporters as “sheep” controlled from abroad.
Medics walked out of hospitals for a second day to join the demonstrations and performers from the Belarusian State Philharmonic held up a message saying “Philharmonia prays for the people”.
Russian internet giant Yandex said armed individuals had entered two of its offices in Minsk and barred employees inside from going out. The intruders left some hours later.