Botswana holds an election on Wednesday that will present the first genuine challenge to the ruling Botswana Democratic Party (BDP) in its five decades of dominance over southern Africa's wealthiest and most stable nation.

Whichever party wins will have to move swiftly to transform the economy, which since independence from Britain in 1966 has grown at 8% a year to become one of Africa's most successful, but now risks coming unstuck because of over-reliance on a single commodity - diamonds.

Duma Boko, leader of the Umbrella for Democratic Change, is hoping to unseat the BDP of President Mokgweetsi Masisi on a promise to do just that. He is backed by former president Ian Khama, who handed over to Masisi last year but has since fallen into a bitter power struggle with him.

"We remain resolute and confident that we are going to win this election," Boko told a news conference on Monday night before warning of possible fraud.

"I can only accept the result if the election is free and credible," he added, raising the spectre that Botswana, which has only known one-sided elections, could witness its first rancorous dispute over a tight result.

Of Botswana's population of 2.2-million people, 924 000 registered voters will elect 57 national assembly and 490 local government representatives.

The winning party's candidate then becomes president.

Polls open at 6:30 am (0430 GMT) and close at 7 pm.

Masisi is standing on his record on tackling corruption, such as making a declaration of assets obligatory for public sector officials, and slashing bureaucracy for small businesses.

Khama, the son of founding president Seretse Khama, quarrelled with Masisi over cabinet posts and the new president's controversial scrapping of an elephant hunting ban in May, which Khama had enforced four years earlier. Elephants, however, have not been an issue during the campaign.

The main concerns for the Batswana are unemployment hovering at around 20% and stark inequalities despite equitable state spending on health and education. To tackle either, the winner will need to do more to diversify the economy.

Diamond mining has fallen to a fifth of GDP in 2018, compared with half in the 1990s, but it still makes up three quarters of foreign exchange earnings, even as banking and tourism grab a greater share of output.