Belarusian leader rejects calls to step down as tens of thousands rally for largest protest yet against disputed vote.

Tens of thousands of opposition protesters have rallied in Belarus's capital for the biggest demonstrations yet against the disputed re-election of President Alexander Lukashenko, even as the leader rejected calls to step down and hold fresh polls.

In a defiant speech at a smaller rally of his own supporters in Minsk's Independence Square, Lukashenko said Belarus would "die as a state if it agreed to hold new elections under pressure" and accused NATO of massing on his country's western border.

"I called you here not to defend me ... but for the first time in a quarter-century, to defend your country and its independence," said the 65-year-old leader.

Belarus has been rocked by a week of street protests after protesters accused Lukashenko of rigging a presidential election last Sunday in which he claimed to have secured 80 percent of the votes.

Lukashenko, who has ruled Belarus for the last 26 years and is facing the greatest challenge to his leadership, denies the allegation.

Belarus protests Tens of thousands of Belarusian opposition supporters gathered in Minsk in the morning to join a 'March for Freedom' over President Alexander Lukashenko's disputed re-election [AFP]

"I am not a fan of rallies but alas, it's not my fault I had to call you to help me," he said as some 5,000 supporters waved national flags and shouted: "Thank you!" and "Belarus!"

Wiping his brow, the president, standing at a podium in a short-sleeved shirt, insisted on the legitimacy of his election victory over popular opposition candidate, Svetlana Tikhanovskaya.

"The elections were valid. There could not be more than 80 percent of votes falsified. We will not hand over the country," he said.

He warned of a threat from neighbouring NATO countries as well as from the opposition movement calling for new elections, as the crowd shouted "No!"

'Lukashenko must answer'

Lukashenko's rally coincided with the opposition movement calling for nationwide "March of Freedom" protests, which saw tens of thousands gathering in Minsk.

Other major Belarusian towns and cities also saw large rallies, local media reported.

Chanting "Leave!", the opposition supporters marched down Independence Avenue, some carrying a red and white flag, once the state flag and now used to represent opposition to Lukashenko's government.

Demonstrators raised victory signs and held flowers and balloons, with many wearing white, the colour that has come to symbolise the opposition movement. Those marching included a group of veteran paratroopers in uniform berets.

"It's unbelievable what I am seeing here right now, there are up to 100,000 people here. Huge crowds," said Vaessen. "This is something Lukashenko was not expecting."

Demonstrators held placards with slogans such as "We are against violence" and "Lukashenko must answer for the torture and dead".

A violent police crackdown on protesters saw more than 6,700 people arrested, hundreds wounded and two people dead. Many protesters alleged they were tortured in custody.

The brutal suppression of post-election protests in Belarus has drawn harsh criticism by Western countries and the United Nations.

European Union foreign ministers said on Friday that they rejected the election results in Belarus and began drawing up a list of officials in Belarus who could face sanctions for their role in the crackdown.

US Secretary of State Mike Pompeo on Saturday said he was glad to see that some protesters in Belarus had been freed but that it was not enough. He said the presidential poll fell short of democratic standards.

On Sunday, Lukashenko spoke to Russian President Vladimir Putin, who told the Belarusian leader that Moscow was ready to provide help in accordance with a collective military pact if necessary, according to a statement issued by the Kremlin.

Lukashenko's challenger Tikhanovskaya, who fled to neighbouring Lithuania on Tuesday, had called for the weekend protests and an election recount. Her campaign announced she was starting to form a national council to facilitate a transfer of power.