Rights group Amnesty International urged Thailand on Friday to open the trial of a political campaigner charged with insulting the monarchy after it was closed for reasons of "national security."

Amnesty said the court's decision to bar the media and public from attending the trial of Darunee Charnchoengsilpakul, a "red shirt" supporter of ousted former premier Thaksin Shinawatra, could jeopardize her chances of receiving a fair hearing.

Darunee, 46, also known as "Da Torpedo," was arrested and charged with lese-majeste last July after delivering an exceptionally strong speech on the 2006 coup that ousted Thaksin.

"When a judge closes the doors on a trial it significantly raises the risk of injustice taking place," Amnesty's Asia-Pacific director, Sam Zarifi, said in a statement.

"The Thai government will have a very difficult time explaining why the trial of someone charged with making an insulting remark could compromise Thailand's national security.

"In this case, a fair trial means that the doors should remain open," Zarifi said.

On Tuesday, Judge Prommas Phoo-sang ordered journalists and Darunee's supporters to leave the courtroom in Bangkok's Criminal Court because her case was a "matter of national security.

In an emotional response to the ruling, Darunee she could not be guaranteed justice if the public were barred from attending. Her lawyer has filed an appeal.

The trial, which resumes on July 28, is the latest in a slew of lese-majeste cases critics say are stifling dissent and freedom of speech.

Lese-majeste, or insulting the monarchy, is a very serious offence in Thailand, where many people regard 81-year-old King Bhumibol Adulyadej as semi-divine and above politics. It is punishable by up to 15 years in prison.

Judge Prommas said he had no comment on the Amnesty statement, but stood by his decision to close the trial.

"One thing I can say, I am impartial," he told Reuters.

(Reporting by Martin Petty and Kittipong Soonprasert; Editing by Darren Schuettler and Sanjeev Miglani)