DUBAI (Reuters) - The son of a detained Iranian opposition leader has been sentenced to six months in jail, his brother said on Monday, after releasing an open letter from his father demanding to be put on trial after years of his house arrest.

Mehdi Karroubi, a former speaker of parliament, has been under house arrest since 2011 with his fellow presidential contender Mir Hussein Mousavi after denouncing the results of 2009 election as rigged, and calling for street protests.

His son, Mohammad Hossein Karroubi, was accused of publishing propaganda against the state over the letter to President Hassan Rouhani in which Karroubi said: “I want you to ask the despotic regime to grant me a public trial.”

In the letter dated April 2016, Mehdi Karroubi added: “I am not asking you to lift my house arrest, as I don’t believe that it is in your power,” but he asked for the public trial “even if the court is constructed the way that the powermongers want.”

Iranian officials have said that the Supreme Leader Ayatollah Ali Khamenei, who has the final say in major state affairs, ordered the house arrest of the opposition leaders.

The former speaker has been accused of being a “seditionist” and “traitor”, but he has not been officially charged.

Mohammad Hossein Karroubi was summoned to the Revolutionary Court some months later for releasing the letter, his brother said, and has been sentenced to six months in jail.

“The main accusation against my brother is that he has emailed that letter to me,” Karroubi’s other son, Mohammad Taghi Karroubi, who lives in Britain, told Reuters by telephone.

He said a copy of his father’s letter was delivered to Rouhani’s office, and a copy was emailed to him that he distributed it among news websites.

He said the family lawyer will appeal against the verdict.

The Iranian judiciary has not commented on the case.

Rouhani promised during his 2013 election campaign that he would try to release the two opposition leaders, but he has rarely addressed the issue in his four years of office.

“Rouhani and his administration have not taken any step to end the house arrest, defend the civil rights, or improve the situation of political prisoners ... He has been mainly focused on economic issues,” Mohammad Taghi Karroubi said.

Rouhani sealed a deal with world powers in 2015 to curb Iran’s nuclear ambitions that lifted most economic sanctions on the country.

Although the opposition movement and its leaders are disappointed with Rouhani for what they say is a failure to expand social liberties, Mohammad Taghi Karroubi said, they will support him in a presidential election in May to “build the future of the country”.