Svetlana Tikhanovskaya, candidate for the presidential elections, greets people during a meeting in Brest. AP

 Polls opened Sunday in Belarus as the ex-Soviet state holds a high-stakes presidential election with a female opposition candidate posing the greatest challenge yet to long-ruling strongman Alexander Lukashenko.

Svetlana Tikhanovskaya, a 37-year-old stay-at-home mother, became an unlikely election candidate after authorities barred her from running and jailed her husband, popular blogger Sergei Tikhanovsky.

Belarusian President Alexander Lukashenko casts his ballot during the presidential election in Minsk on Sunday. AP

She swiftly emerged as Lukashenko's strongest rival and her rallies drew tens of thousands of supporters across the country.

Her presidential bid gave rise to an informal new protest movement, with many dreaming of change in the country of 9.5 million people led for the last 26 years by 65-year-old Lukashenko.

Political observers said there is little doubt that Lukashenko, who is seeking a sixth term, would rig Sunday's vote in the absence of international observers. 

A man prepares to cast his ballot at a polling station during the presidential election in Minsk. AP

More than 41 per cent of votes had already been cast in several days of early voting, the central electoral commmission reported, with observers saying this facilitated falsifications.

Voters wearing face masks began casting ballots at a Minsk polling station early Sunday, an AFP reporter saw. Some wore white bracelets after Tikhanovskaya her supporters to wear them.

There was a heavy police presence after detentions of protesters on Saturday evening and Prosecutor-General Alexander Konyuk urged voters to be "reasonable" and not take part in unsanctioned protests, Belta state news agency reported.

Tikhanovskaya, who joined forces for her campaign with the wife of one barred opposition chief and the campaign manager of another, said she would not call on her supporters to protest after the vote.

At the same time she called on law enforcement to refrain from fulfilling "criminal orders" and has urged election officials to count honestly. 

A woman walks along a street and raises her hand with a white ribbon, a symbol of Belarus' opposition movement. Reuters

Belarus has not held polls judged free and fair since 1995 and this time Minsk has not invited observers from the European OSCE observer group for the first time since 2001.

In the past Lukashenko has crushed protests with riot police and hefty jail terms, prompting Western sanctions. Two of his would-be election rivals including Tikhanovskaya's husband were jailed during the current campaign.

Tikhanovskaya says she is not a politician and if she wins she will call fresh elections that will include the entire opposition. On Saturday, authorities arrested her campaign manager and briefly detained one of her two top allies.

Many Belarusians said they hoped polls would lead to change.

Agence France-Presse