The UN human rights chief on Wednesday condemned the violent crackdown following the disputed presidential election in Belarus and called for the immediate release of unlawfully detained protesters.

Michelle Bachelet, the United Nations High Commissioner for Human Rights, decried the reported police use of rubber bullets, water cannon and stun grenades on demonstrators, and said their grievances must be heard.

"I remind the Belarusian authorities that the use of force during protests should always be exceptional and a measure of last resort, clearly differentiating between any violent individuals and peaceful protesters, against whom force should not be used," she said in a statement.

"People have the right to speak up and express dissent, even more in the context of elections, when democratic freedoms should be upheld, not suppressed."

Protests broke out after authorities claimed President Alexander Lukashenko, who has been in power since 1994, won 80 percent of the vote in Sunday's polls.

Police in Belarus said Wednesday they had shot at protesters with live ammunition in a third night of violence, while more than 6,000 people have been detained overall.

Bachelet lambasted the "trend of massive arrests in clear violation of international human rights standards.

"Even more disturbing are the reports of ill-treatment during and after detention," she said, reminding the authorities in Minsk of the absolute prohibition on torture and ill-treatment of detainees.

She called for the immediate release of those unlawfully detained, and swift, thorough and impartial investigations into alleged human rights violations.

"Those arbitrarily detained or ill-treated for peacefully expressing dissent are entitled to justice and redress," the former Chilean president said.

"The authorities should also hear and respond to people's grievances regarding the elections."

Bachelet also voiced concern at the intermittent internet shutdowns since Sunday, as well as the blocking of many social media platforms, saying it amounted to a severe curtailment of the right to freedom of expression.

Riot police have also targeted press photographers, pulling out memory cards and breaking cameras.

"Free flow of information is crucial in any democratic society, and especially in a context of crisis and social unrest," said Bachelet.

"But even more so, in the current situation of the COVID-19 pandemic, and where people might feel compelled to express dissent online rather than on the streets. The right to peacefully protest online must be also protected."