By Ivan Castro

GRANADA, Nicaragua (Reuters) - A U.S. citizen serving a 22-year prison sentence in Nicaragua for drug trafficking and money laundering, who a United Nations group has said was wrongly convicted, appeared in court on Thursday for the start of a long-awaited appeals proceeding.

In a brief statement before the court, Jason Puracal maintained his innocence, saying he hoped justice would prevail.

Puracal, 35, was detained by Nicaraguan authorities in November 2010 along with 10 Nicaraguans and accused of transporting drugs to El Salvador and Guatemala.

He was later found guilty by a trial judge along with his co-defendants, despite their testimony that they had never met or worked with Puracal, his legal team has said. It added that the prosecution's own witnesses said he was innocent.

Puracal has become a cause célèbre for human rights activists in the United States and around the world, with U.S. lawmakers appealing to Nicaraguan President Daniel Ortega and a former high-ranking U.S. Drug Enforcement Administration official launching a massive petition drive on Puracal's behalf.

At the hearing, the court admitted as evidence a letter written by the U.S. Drug Enforcement Administration that confirmed the agency was not investigating Puracal. Prosecutors had objected to the move.

"This letter has no relevance to this case," said prosecutor Rodrigo Zambria, the deputy head of the government's anti-corruption and organized crime task force.

The proceedings were expected to last a week, and a decision could come anywhere from five days to months after the hearing concludes.

Supporters have been pushing for the appeal to be heard for nearly a year, and heightened those efforts earlier this month after learning that Puracal, who has been in solitary confinement, was put on suicide watch by Nicaraguan authorities.

The United Nations Working Group on Arbitrary Detention said in May that Puracal was arbitrarily imprisoned and recommended he be immediately freed.

A U.S. citizen born in Washington state, Puracal became a resident of Nicaragua after serving there as a Peace Corps volunteer in 2002, and he has married a Nicaraguan woman with whom he has a son.

Before his arrest, he was working at a real estate office in the Nicaraguan city of San Juan del Sur, a surfing destination on the Pacific Coast. Puracal's supporters said he came under suspicion due to his job as a real estate agent, which gave him control over large sums of money held in escrow for property transactions.

The prosecution holds that Puracal used a real estate company to buy properties with drug money.

(Writing by Gabriel Stargardter; Editing by Cynthia Johnston and Lisa Shumaker)