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Bahrain's most prominent opposition figure faces four years in prison on speech-related charges following an unfair trial, on Tuesday.

Sheikh Ali Salman was convicted on charges that violate the right to freedom of expression reported the human rights organisation. It called for authorities to release him immediately.

Salman was convicted on 16 June 2015 by Bahrain's Fourth Superior Criminal Court and sentenced him to four years in prison.

His appeal is scheduled for September 15.

During the trial the judge "refused to allow defence lawyers to present potentially exculpatory evidence, including recordings of speeches he was prosecuted for, on the grounds that 'the intent of them is to raise doubts about the substantiating evidence that has persuaded the court,'" reported HRW after reviewing the trial documents.

"The court's refusal to consider crucial defence evidence confirms the political nature of Sheikh Ali Salman's prosecution," said Sarah Leah Whitsom, HRW's Middle East director.

The judge did not allow Salman's lawyers to present video or audio evidence of the allegedly offending speeches and sermons in court.

"The manifest unfairness of the trial means the authorities should release Salman immediately," she added.

Salman, who is secretary-general of Al Wifaq, the largest recognised opposition general society in Bahrain, was arrested on 29 December 2014 and has been in detention ever since.

According to Bahrain's chief prosecutor, Nayef Yousef Mahmoud, he was detained due to "a series of recent statements and pronouncements".

However, even though the charges concerned his speeches and sermons, the judge did not allow Salman's lawyers to present video or audio evidence of the allegedly offending speeches and sermons in court.

His lawyers received no response when they wrote to the chief justice of the court in February asking for the trial to be held in a venue with audio-visual equipment.

The court failed to rely on the actual content of his speeches and instead listened to the testimony of Khalid al-Sa'idi, an interior ministry officer who said he had listened to recordings of the sermons and speeches.

Al Sa'idi's testimony, that formed the basis of the prosecution's case against Salman, may have misrepresented the speeches' content.

For example, al-Sa'idi claimed that during a sermon on 10 October 2014, Salman said: "The people have bigger and bigger force in them. All that you need to do is call forth this force. I'm talking now about military force."

The  shows Salman actually said "I am not talking about military force".

By refusing to allow actual recordings of the speeches to be submitted as evidence the judge has fundamentally violated Salman's right to a fair trial.

"Bahraini authorities should release him immediately and either drop the charges or retry Salman on any charges that do not themselves violate the right to freedom of expression or other basic rights," Human Rights Watch said.