ARUSHA, Tanzania — A U.N. appeals court on Monday overturned the conviction of the former Rwandan president's brother-in-law, who had been sentenced to 20 years for organizing a massacre that left about 1,000 dead during the 1994 genocide.

The judge said that serious errors had been committed during Protais Zigiranyirazo's 2008 conviction and sentencing, and ordered him to be released immediately.

Zigiranyirazo, 71, stood in disbelief in the courtroom on Monday. "God is great and justice has been done," he told The Associated Press after the judge overturned the sentence.

At least 500,000 ethnic Tutsis and moderate Hutus were killed during Rwanda's genocide, which began after President Juvenal Habyarimana's plane was brought down in April 1994. Zigiranyirazo, an influential member of the Rwandan government at the time, was his brother-in-law.

In a 30-page ruling, the appeals court said Monday that it had reversed Zigiranyirazo's convictions for genocide and crimes against humanity because those convictions had "violated the most basic and fundamental principles of justice."

"In these circumstances, the Appeals Chamber had no choice, but to reverse Zigiranyirazo's conviction," the ruling said.

Judge Theodor Meron said the trial judgment had "seriously erred in its handling of the evidence."

Lead defense lawyer John Philpot said that Zigiranyirazo, known as "Mr. Z," should be sent back to Belgium where he was arrested, or to France where his wife lives.

"I am extremely happy with the judgment, but the damage done must be reimbursed by the prosecution for the 8<0x00BD> years spent in detention by Mr. Zigiranyirazo," Philpot said.

According to the indictment, Zigiranyirazo was accused of leading a convoy that attacked Tutsis who were seeking refuge on a hill a few days after the genocide began. About 1,000 people were killed and the convoy later returned to attack survivors, the indictment said.

Zigiranyirazo also was accused of ordering people to set up roadblocks as part of a campaign to kill Tutsis and of paying people to dig a mass grave.

The International Criminal Tribunal for Rwanda, set up by the U.N. to try key suspects of the genocide, has convicted 39 people and acquitted seven. Trials are under way for 11 others, and 11 most-wanted fugitives are still on the run.

The 1994 genocide began after Habyarimana's plane was brought down by unknown assailants as it was approaching the Rwandan capital, Kigali. Burundi's President Cyprien Ntaryamira also was killed. Both were returning from a regional peace meeting in Dar es Salaam, Tanzania.

The killings spread across the country and lasted 100 days, until a Tutsi, Paul Kagame, led his rebel army to overthrow the Hutu government. Kagame is now Rwanda's president.