STOCKHOLM—A Swedish court of appeal on Thursday upheld a life prison sentence for Stanislas Mbanenande, the first person in Sweden to be convicted of genocide, for crimes committed during the Rwandan genocide of 1994.

The 55-year-old Swedish citizen of Rwandan origin was first sentenced by a Stockholm district court in June last year for a string of crimes, including murder and abduction, in connection with a large number of massacres in the Kibuye prefecture of western Rwanda.

Mr. Mbanenande has denied all charges, and his defense team claimed in court that the Rwandan regime fabricated the investigation and wrongfully accused Mr. Mbanenande. The appellate court said that it found no reason to change the district court’s verdict, arguing that testimony from victims and witnesses was reliable even though the events took place 20 years ago during emotionally difficult and chaotic conditions.

“The Court of Appeals has concluded that the identifying testimonies, despite the long time that has passed, are so reliable that it is clear it is the defendant,” the Svea Court of Appeal, in Stockholm, said in a statement.

The defendant’s lawyer, Tomas Nilsson, said that Mr. Mbanenande would likely appeal to the supreme court. “We’re very critical of this ruling and believe the court of appeal hasn’t taken into account the particular difficulties regarding the evidence in this case,” he said.

“The case rests only on oral testimonies and there are still feelings of revenge. There’s a risk that witnesses have been influenced, such things happen in Rwanda which isn’t exactly governed by the rule of law,” he added

The case covered multiple crimes at several different occasions. The appeal court said that it had overturned the verdict in one instance because of uncertainty over the perpetrator’s identity, but that it didn’t change the life sentence.

Mr. Mbanenande was a civil engineer and university lecturer when the crimes were committed. He and his family left Rwanda because of the civil war and he eventually moved to Sweden in 2007. He became a citizen in 2008.

The eight-month appeal trial was partly carried out in Rwanda where the judges visited various crime scenes. The district court stated in its verdict that Mr. Mbanenande served as an informal leader in Rwanda and on several occasions fired an automatic weapon into groups of people.

The Rwandan genocide involved mass-killings targeting the Tutsi minority and moderates belonging to the Hutu majority. Approximately 800,000 people were killed over the course of about 100 days, according to the United Nations, although estimates vary greatly.

In Sweden, felons sentenced to life in prison can apply to have their sentences converted into a fixed term after serving 10 years, an option that most convicts are eventually granted. A life sentence can’t be converted into a fixed-term sentence shorter than 18 years. End

By Anna Molin at anna.molin@wsj.com