A former interior minister and enforcer for a violent and autocratic Gambian president was convicted of crimes against humanity on Wednesday for the torture and executions of civilians and sentenced to 20 years in prison by Switzerland’s federal court.

The verdict, which one plaintiff called a “milestone” for victims, came after a landmark trial that was followed closely by victims of the government’s repression.

The former minister, Ousman Sonko, 55, was found guilty of multiple counts of intentional homicide, torture and false imprisonment that were committed, the court said, as “part of a systematic attack on the civilian population” of the West African country.

His lawyer said he would appeal the verdict.

Mr. Sonko, who moved to Switzerland in 2016 and has been in custody there since he was arrested in 2017, when a human rights group based in Geneva filed a criminal complaint against him, will serve 13 more years in prison and then face deportation to Gambia. The case was tried in Switzerland under the legal principle of universal jurisdiction, which allows states to prosecute serious crimes regardless of where in the world they were committed.

Mr. Sonko had held a series of powerful security jobs under Yahya Jammeh, an eccentric autocrat who ruled Gambia for 22 years before fleeing into exile to Equatorial Guinea after losing an election in 2017.

Mr. Sonko rose from commander of the presidential guard to police chief and then to interior minister, a post he held from 2000 to 2016. During that period, the court said, political opponents, journalists and critics of the government “were routinely tortured, executed extrajudicially, arbitrarily arrested and detained.”

Prosecutors accused Mr. Sonko of participating in the killing of a soldier suspected of plotting a coup, Almamo Manneh, and of beating and repeatedly raping Mr. Manneh’s widow, Binta Jamba. He was also accused of torturing an opposition party leader, Ebrima Solo Sandeng, who died in state custody in 2016.

The Swiss court did not consider that his offenses had amounted to aggravated crimes against humanity, which could have earned him a life sentence, but it handed him the maximum possible term in prison for the lesser charge of non-aggravated crimes.

The court also did not rule on the charge of rape despite the testimony of Ms. Jamba that he had violently raped and tortured her. The charges were dropped, as the court considers it an individual crime that is outside its jurisdiction.

Annina Mullis, who represented Ms. Jamba, said the decision was part of a wider pattern of courts disregarding rape as part of systematic violence.

“It’s disappointing that the court failed to take this chance to recognize sexual violence as a tool of repression,” she said.

Benoit Meystre, a lawyer for TRIAL International, the legal advocacy group based in Geneva that initiated the case against Mr. Sonko in 2016, described the verdict as “historical.”

European courts have tried a number of individuals for crimes under universal jurisdiction in recent years, but Mr. Sonko, as a former government minister, is the most senior state official to be prosecuted, Mr. Meystre said, serving notice that rank is not a guarantee of impunity.

Fatoumatta Sandeng, a plaintiff in the case and the daughter of the tortured opposition leader, was in court to hear the verdict. Afterward, she said in a statement: “I am very happy and relieved. The judgment is an important milestone for us victims.”

She also said that “it was good to hear” that the court had finally recognized that Mr. Sonko had been responsible for her father’s death.

Her lawyer, Nina Burri, expressed regret that the court had not considered the sexual violence charge as a crime against humanity but called the verdict “an important step in the fight against impunity” that showed even the highest-ranking officials “cannot hide and will be held responsible.”

Philippe Currat, the lawyer for Mr. Sonko, said in a telephone interview on Wednesday after the verdict, “We will certainly have a second round.”

Mr. Currat said the court had failed to distinguish between Mr. Sonko’s individual role in events and the part played by other actors. “It is not because he is a minister that he is responsible for everything that happened in the country,” the lawyer said.

Mr. Sonko, in his defense, said that he had sought to professionalize the police and was never in charge of the National Intelligence Agency, which had detained and tortured protesters, including Mr. Sandeng, the opposition leader.

Gambian activists said they hoped that Mr. Sonko’s trial would spur the government of President Adama Barrow to take long-promised action on victims’ demands for accountability for the crimes of the Jammeh era.

Other plaintiffs in Gambia hailed Wednesday’s verdict.

“Justice has finally come,” said Madi Ceesay, a journalist who was arrested and tortured in 2006, after he wrote a column criticizing coups, including the one in 1994 that brought Mr. Jammeh to power. Mr. Ceesay’s newspaper, The Independent, was also shut down.

Because Mr. Sonko and Mr. Jammeh wielded such power, he said, “I’ve never thought a day like this could come.”

Mr. Ceesay said that while he considered Mr. Sonko “the man at center stage” in connection with his own arrest and torture, Mr. Jammeh should face justice, as well.

“He’s the biggest fish,” he said of Mr. Jammeh.

Mr. Sonko’s conviction was a lesson to dictators everywhere that they would eventually be held accountable, he said, adding, “There’s nowhere you can hide in the world.”