NAIROBI, Kenya — Ugandan security forces on Friday surrounded and then breached the compound of Bobi Wine, the country’s leading opposition candidate, a day after a contentious general election that he said was marred by widespread “fraud and violence.”

The breach, reported by Mr. Wine and confirmed by several people involved in his campaign, took place as the country’s electoral commission released partial results of the general election that showed the incumbent president, Yoweri Museveni, in the lead. Mr. Wine’s lawyer said the siege effectively constituted house arrest.

Mr. Wine, 38, was the most potent challenger to Mr. Museveni, a 76-year-old who has ruled the country for 35 years. The tense election campaign was marked by a crackdown on opposition figures like Mr. Wine and others, which sparked nationwide protests that were put down by police and resulted in the killing of more than 50 people. An internet shutdown that started just before Election Day is still in place.

With ballots from almost half of the country’s polling stations counted, preliminary results show Mr. Museveni with more than 62 percent of the vote and Mr. Wine with 29 percent, according to the country’s electoral commission.

On Friday afternoon, Mr. Wine said that forces with the Ugandan military along with plainclothes officers carrying guns broke into his compound in the capital, Kampala.

“We are under siege,” Mr. Wine, a musician-turned lawmaker whose real name is Robert Kyagulanyi, said in a post on Twitter. “The military has jumped over the fence and has now taken control of our home.”

“None of these military intruders is talking to us,” he added in another tweet. “We are in serious trouble.”

Spokesmen for the government and the Kampala police did not immediately respond to requests for comment.

The news of the break-in was confirmed by Jeffrey Smith, founder of Vanguard Africa, a nonprofit based in Washington that has worked with Mr. Wine for three years.

In an interview, Mr. Smith said he got a call from Mr. Wine after 4:30 p.m. Kampala time during which he heard “lots of shouting and banging.” During the call, which lasted five minutes, Mr. Wine told him that security officers had assaulted some of his staff members and arrested a gardener, Mr. Smith said.

Bruce Afran, Mr. Wine’s lawyer, later said that by surrounding his home, the government was placing him “under house arrest.”

“The military are registering anyone who enters his house and inspecting vehicles as they leave to be sure he is not inside and leaving the property,” he said in an interview.

Mr. Wine has had numerous confrontations with security forces, even before he filed his candidacy for president last November.

In 2018, Mr. Wine was arrested and beaten by security forces and left for the United States to seek medical treatment. On the campaign trail, Mr. Wine was arrested and charged with breaching coronavirus rules and was pulled out of his car while speaking in an online news conference.

The day before the election, authorities forced his private security guards to withdraw from protecting his home, Mr. Afran said.

He filed a petition in the International Criminal Court in early January accusing top government officials of sanctioning a wave of violence and attempting to kill him.

In a news conference earlier on Friday at his residence, Mr. Wine sounded upbeat about his prospects of winning and cast doubt on the early results.

“We have certainly won this election and we have won it by far,” Mr. Wine said. “The people of Uganda will and must reject the blatant usurpation of their will and their voice.”