Human rights activists and monitors say there is growing evidence of police brutality against people detained in Belarus as protests over the contested election continue.
Some of the 6,700 people detained since Sunday have been released and are being subjected to abuse, 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 President Alexander Lukashenko long-term opt for another term.
In a statement on state television, Senate Spokeswoman Natalya Kochanova said the president had ordered an investigation into the mass arrests of Protestants, and that more than 1,000 had been released.
‘Torture rooms’
Some of the released prisoners shared photos on messaging app Nexta showing their injured and swollen bodies, including injuries to back and buttocks, which were presumably caused by police.
Amnesty International said people arrested were stripped naked, beaten and threatened with rape.
“Former detainees have told us that detention centers have become torture chambers, where Protestants are forced to lie in the dirt while police beat and fight with truncheons,” said Marie Struthers, director of Amnesty International for Eastern Europe and Central Asia. .
In audio shared by a BBC journalist, screams could be heard from inside Okrestina detention center in the capital Minsk.
Separately, a group of five UN human rights experts said the security forces’ response to peaceful protests had been violent, with frequent use of excessive, unnecessary and indiscriminate force, reports AFP news agency.
“Authorities only seem interested in spreading the protests quickly and arresting as many people as possible,” they said in a joint statement.
Interior Minister Yuri Karayev said he had taken responsibility for people injured during the protests, and that he apologized to those affected by the violence.
Observers say the official statements and the release of the detainees suggested a more conciliatory approach following public outcry and international condemnation in the police response. EU foreign ministers had to consider imposing sanctions on Belarus.
Officials have confirmed two deaths during the unrest. One protester died during a protest in Minsk on Monday, while a man died after being arrested on Wednesday in the southeastern city of Gomel.
Shock at police brutality as witnesses mount
By Olga Ivshina, BBC Russian
The body of evidence of police brutality, both in the streets and in prisons, is on appeal. Detainees include not only opposition activists but also many journalists and passers-by.
One of the released journalists, Nikita Telizhenko of the Russian news website Znak.com, published a harrowing of three days in prison. Now back in Russia, he describes people lying on the floor of a detention center, stacked 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 people injured, with broken limbs and severe bruises, not only without medical help, but more being kicked and beaten by the guards.
Telizhenko’s testimony is confirmed by countless posts on social media – photos, videos, stories. I spoke to an American woman who visited her Belarusian friend in Minsk – he was detained for no apparent reason. Not only had he not protested, but he was sleeping in bed when the police came to his apartment, put the door down and took him away.
More on the protests in Belarus
What else happened Thursday?
Footage posted by messaging app Nexta appeared on Thursday night to show protests going on in the streets with protesters singing “out”.
Tired relatives gather outside a prison north of the capital Minsk, hoping to reunite with their loved ones or get information about their whereabouts.
Several strikes have been reported at factories in the state, where workers are protesting against the harsh treatment of Protestants. Hundreds of employees were seen running at truck-maker Belaz in Zhodino to the north-east of the capital.
Employers Minsk and Grodno in the west also organized strikes and walks. 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 attacking the female Protestants in Minsk, with a bunch of flowers. She was one of three women who gathered their resources to spare the opposition. The other two have left the country.
Veronika Tsepkalo fled to Belarus on polling day, while the main opposition candidate in the elections, Svetlana Tikhanovskaya, was briefly detained on Monday before being forced to leave Lithuania.
Ms Tikhanovskaya, 37, released a video saying she made the “very difficult decision” to leave because of her children.
The opposition candidate was mothered as a stay-at-home mom until she entered the race after her husband was arrested and blocked from registering for the vote.
She became Mr Lukashenko’s most expensive opposition challenge in years, leading to major rallies in the run – up to the vote.
But Mr Lukashenko rejected her offer, saying that a Belarusian woman could not lead. At the age of 65, he ruled the former Soviet Union since 1994, describing opposition killers as “sheep” controlled from abroad.
Medics rushed out of hospitals for a second day to attend demonstrations and Belarusian Philharmonic State officials held up a message saying “Philharmonia prays for the people”.
Russian internet giant Yandex said armed people entered two of its offices in Minsk and employees were confused to leave. The burglars left a few hours later.