Botswana President Mokgweetsi Masisi announced Friday that he would "step aside" following a significant defeat in the general elections, according to preliminary results.

"I wish to congratulate the opposition on their victory and concede the election," Masisi told reporters during a press conference, describing it as "a good journey."

His party, the Botswana Democratic Party (BDP), has governed the diamond-rich nation since its independence from Britain in 1966.

Results from Wednesday's vote are expected to be confirmed by the electoral commission later Friday, but early counts show that three opposition parties together won at least 31 of 61 seats in the national legislature.

Under Botswana's electoral system, the first party to reach 31 seats will be declared the winner and install its candidate as president.

Masisi, who took office in 2018, said he would "begin all administrative work to facilitate the transition."

"We are quite happy to retreat into being a loyal opposition and to hold the government accountable," said the 63-year-old leader, who had been confident of securing a second term.

The left-leaning opposition Umbrella for Democratic Change secured more than 24 seats, party official Mike Keakopa told Agence France-Presse (AFP), and was aiming to reach 31 seats to become the outright winner.

If confirmed, its candidate, the Harvard-educated human rights lawyer Duma Boko, would be the next president.

The two other opposition parties, the Botswana Congress Party and Botswana Patriotic Front, together took around a dozen seats.