Police raided the headquarters of Uganda’s main opposition party and arrested its leader again Friday, as controversy deepened in the vote count to decide the East African nation’s next president.

Police fired tear gas and rubber bullets to disperse officials of the main opposition Forum for Democratic Change, who were set to hold a news conference and accuse supporters of incumbent President Yoweri Museveni of vote rigging in Thursday’s presidential and parliamentary elections.

They also detained the opposition party’s leader, Kizza Besigye, for the second time in two days, as well as several other members of the party, police and witnesses said. He was still in police custody late Friday, and no charges had been filed against him, his lawyer said.

With all official results expected Saturday, early vote tallies released by Uganda’s official election commission indicated Mr. Museveni would get another five years in office. He first became president in 1986.

The incumbent was leading with 62.8% to 32.7% for Mr. Besigye, with 45% of the ballots counted, the election commission said Friday.