A Thai judge citing reasons of national security closed the trial on Tuesday of a "red shirt" supporter of former premier Thaksin Shinawatra charged with insulting the monarchy.

The ruling drew an emotional response from the defendant, Darunee Charnchoengsilpakul, who was arrested last July after delivering an exceptionally strong speech on the 2006 coup that ousted Thaksin.

"I want justice," the 46-year-old campaigner, better known as "Da Torpedo", told the judge before a handful of journalists and supporters were ordered to leave.

"The speech I am charged with was made at an open rally. I cannot accept that a closed trial will guarantee justice," she said, standing before the court wearing a dark prison gown.

The trial, which was due to hear testimony from police investigators on Tuesday, 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 Prommat Toosang said Darunee's trial at Bangkok's Criminal Court was a "matter of national security" but her lawyer could appeal against the decision.

"For now, anybody not involved with this case needs to leave the courtroom immediately," he said.

"I guarantee the defendant will get a fair trial, but this has to be a closed-door hearing."

Speaking to Reuters before the hearing, Darunee said she had lost 17 kilos (37 lb) after nearly a year in detention awaiting trial.

"My life in jail is hard," she said, holding out little hope of winning her case after being denied bail several times.

She said her speech at a pro-Thaksin rally in Bangkok last year was aimed at the generals who ousted the former telecoms tycoon, who lives in self-imposed exile after his conviction on conflict of interest charges.

"I do not want to topple the monarchy in Thailand. What I want is a sustainable monarchy like in the United Kingdom and Japan," Darunee said.

Critics of the lese-majeste law say it is open to abuse since a complaint can be filed by anybody against anybody, no matter how minor the alleged disrespect.

The law has been a regular feature of the charged political atmosphere in Thailand in the past three years. The generals who overthrew Thaksin cited his alleged disrespect for the monarchy among other reasons.

Prime Minister Abhisit Vejjajiva has said he wants to strike a balance between upholding the law and freedom of expression, but critics say little has changed.

In April, a Thai man was jailed for 10 years for posting comments on the Internet deemed insulting to the monarchy.

A month earlier, a court issued an arrest warrant for a leading Thai political analyst who fled to London after being charged with insulting the king in a book published in 2007.

Prosecutors are also deciding whether to pursue lese-majeste charges against Jakrapob Penkhair, a former minister in a pro-Thaksin government, for allegedly insulting the king in a speech in 2007. Foreigners who run afoul of the law usually receive a royal pardon. An Australian author sentenced to three years in prison for defaming the crown prince in a little-read novel was sent home in February after several months in jail.

(Additional reporting by Kittipong Soonprasert)