Botswana’s main opposition leader says October’s national election, which the ruling party won by an unexpectedly large margin, was rigged, and that he intends challenging it in court to head off potential public protests.

Duma Boko, leader of the Umbrella for Democratic Change opposition alliance, questioned the integrity of the voters’ roll. He said many of his group’s supporters, including his own wife, were turned away at the ballot box due to errors made by the Independent Electoral Commission (IEC). The number of voters was also inflated, he said.

President Mokgweetsi Masisi’s Botswana Democratic Party extended its 53-year grip on power by winning 38 out of 57 constituencies in the October 23 vote. The party, which has been in power since Botswana won independence from the UK in 1966, rebounded from its worst-ever electoral performance in 2014. That was despite infighting within its ranks and a host of social ills ranging from inequality to a high rate of HIV-prevalence. The UDC won 15 seats, compared with 17 seats in 2014.

“We have to put our faith in the courts,” Boko said in an interview at Bloomberg’s Johannesburg office on Tuesday, adding that he’s concerned about the potential reaction of his supporters. “If people lose faith in the electoral process, then what steps will they take to change government?”

Boko said there was a 33% discrepancy between the number of people the IEC said were on the roll in March and the actual total when counted. About 75,000 people, or 8% of the electorate, didn’t have their gender recorded on the roll, disqualifying them from voting, he said.

“We accept a certain amount of human error when carrying out a manual process, but the extent of this error can only be attributed to deliberate tampering,” he said. “Is there any other explanation?”

The election was endorsed by observers from the African Union and the Southern African Development Community.

“Mr Boko is a politician and obviously he would make political statements, but international observers have declared that the elections were free and fair,” said Andrew Sesinyi, a government spokesperson. “In the country there is general consensus that they were free and fair.”

Some analysts said Masisi re-energised the BDP’s support base and benefited from splits in the opposition vote.

Boko also said the low turnout in former opposition strongholds in the capital, Gaborone, compared with the 83.8% turnout nationally, was suspicious. The opposition lost seats it has usually won in the past, including his own.

He has 30 days from the date of the election to file legal objections and hasn’t yet decided whether to challenge outcomes in individual constituencies or the entire result. While there have been legal challenges to constituencies in previous polls, national results have never been questioned in court.

“The laws of Botswana allow him or any person to challenge the outcome of an election result,” said Osupile Maroba, a spokesperson for the IEC. The commission is aware of Boko’s concerns, he said.

While Botswana’s economy is expanding, with growth expected to average 4.2% until 2024, according to the International Monetary Fund, its key diamond industry has limited employment opportunities and its tourism industry is under threat from a controversy over elephant hunting.