Protesters marched to the Belarusian government building in Minsk on Friday after widespread claims that prisoners freed on Friday after days of detention had been tortured in prison.
Hundreds have been released from jail after being interned during the violent state crackdown on demonstrations against President Alexander Lukashenko's controversial re-election last Sunday.
Opposition leader Sviatlana Tsikhanouskaya has accused the authorities of creating a "bloodbath", and has called for more rallies this weekend.
The Belarusian government announced on Thursday night that 1,000 people arrested during the protests were being let go. However, many people were still waiting for news of missing friends and relatives on Friday morning.
Testimony describing serious human rights abuses by police and prison guards is growing. Amnesty International says there is mounting evidence of the widespread torture of peaceful protesters.
Several newly-freed protesters have described how they were deprived of water and sleep, given electric shocks and burned with cigarettes, and beaten by guards. Dozens of detainees are said to have been crammed into tiny cells designed for only a handful of people.
There were emotional scenes as dozens of detainees were let out of the Okrestina prison in Minsk at around midnight on Thursday. Volunteers also saw over 100 people being freed at Zhodino just outside the capital.
Ambulances arrived to carry those who apparently were unable to walk on their own.
Opposition leader condemns 'bloodbath'
Tsikhanouskaya, who fled to nearby Lithuania this week for her safety and is demanding an election recount, condemned the crackdown in a video address on Friday.
"The authorities have turned the peoples' peaceful protest in the streets into a bloodbath. The situation is becoming critical. It is painful to see what is happening in our country in the past few days," she said.
Stressing that protests should be non-violent, she called on the authorities to end the violence and enter a dialogue, asking city mayors to organise "peaceful, mass rallies" on Saturday and Sunday.
The 37-year-old former teacher joined the election race to replace her husband, an opposition blogger, who has been jailed since May.
EU foreign ministers are due to hold a special meeting on Friday by video link to consider what to do about Belarus, amid calls for sanctions to be imposed.
More join protests despite repression
The streets of the capital Minsk were reportedly quieter on Thursday night, but thousands of people are still said to have come out to denounce the repression against the protest movement. The past two days have seen hundreds of women join the movement, forming human "chains of solidarity".
Hundreds of academics, IT executives, musicians and factory workers are among those who have added their voices condemning the violence. Several strikes have been reported among workers in state industries.
Videos have been posted apparently showing police and military personnel dumping their uniforms, while several presenters at Belarus' state TV stations have quit.
Nearly 7,000 people have been detained and hundreds injured in the clampdown on demonstrators protesting the official results that said Lukashenko won 80% of the vote and his top opposition challenger Tsikhanouskaya got only 10%. There is a widespread belief that the election was rigged.
Police have broken up protests with stun grenades, tear gas, rubber bullets and severe beatings -- although on Thursday night they are said to have stood back.
Euronews has been reporting from Minsk on claims that prisoners have been beaten and mistreated amid appalling conditions in detention centres.
Authorities respond to pressure
The Belarusian Foreign Minister Vladimir Makei has said his government is ready for a "constructive and objective dialogue" with other countries, on all events in Belarus concerning the election.
The move from the authorities to release prisoners comes amid widespread revulsion at the violent repression, at home and abroad.
"More than 1,000 people have been released on condition that they do not take part in unauthorised demonstrations," Senate Speaker Natalia Kochanova told state television on Thursday night.
Interior Minister Yuri Karaev apologised for the police violence committed against "passers-by" not involved in the protests.
Lukashenko, in power for 26 years in the ex-Soviet state, has never allowed any serious opposition movement to take root in Belarus. The last significant wave of protest in 2010 was harshly suppressed.