DAKAR (Reuters) - A Gambian opposition leader on trial since April for participating in a protest has said he is a victim of political persecution and will boycott the proceedings.

Ousainou Darboe, a lawyer, was arrested after taking part along with other senior members of the United Democratic Party (UDP) in a small protest near the capital Banjul.

The UDP said 50 people had been arrested after the protests in two round-ups, which the United Nations and the United States condemned. It says it fears that three of the detainees are dead, including senior party official Solo Sandeng.

Twenty UDP officials including Darboe are being tried for holding a protest without a permit.

Protests are rare in Gambia, a sliver of territory almost encircled by Senegal, where President Yahya Jammeh has ruled for two decades since seizing power in a bloodless coup.

Rights groups say he has been cracking down on opposition figures with an eye on December's election, where he will seek his fifth term, having scrapped term limits; Jammeh once told a journalist he could rule for "a billion years".

Lawyers for Darboe and for the other party leaders walked out of the courtroom on Thursday.

"This is a political case, a case against me and my party supporters," Darboe said on Thursday. "We will not participate in this trial to legitimise our pre-arranged conviction."

A police officer told the court on Thursday that protesters had refused to disperse and started throwing rocks, at which point the police fired tear gas at them.

The U.S. Senate held a hearing on Wednesday in which the question of imposing sanctions on Gambian leaders was raised. The next day, the Gambian government withdrew police protection from the American embassy, the embassy said.

(Reporting by Makini Brice; Editing by Kevin Liffey)