BANGKOK - A Thai court on Friday sentenced a political campaigner to 18 years in prison for insulting the monarchy, the latest in a slew of lese-majeste cases critics say are stifling dissent and freedom of speech.

The Bangkok Criminal Court handed the harsh sentence to Darunee Charnchoengsilpakul, a red shirt supporter of ousted former premier Thaksin Shinawatra. She was convicted of three counts of royal insults, each with a six-year sentence.

The court finds she intended to insult and make threats to the king and the queen, one of the three judges said, reading the verdict.

Darunee, 46, also known as Da Torpedo, was arrested in July last year after delivering an exceptionally strong speech about the 2006 coup that ousted Thaksin.

It is what I expect to happen, she told reporters after the verdict. I will appeal.

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.

The trial, which began in June, had been held behind closed doors for reasons of national security, which rights group Amnesty International said could jeopardize her chances of receiving a fair hearing.

Prior to her first hearing, Darunee said her speech was aimed at the generals who ousted former telecommunications tycoon Thaksin, who lives in self-imposed exile after his conviction on conflict of interest charges.

She told Reuters she had made no attempt to topple the monarchy and that she supported a sustainable monarchy like in the United Kingdom and Japan.

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.

(Writing by Martin Petty; Editing by Jason Szep and Sanjeev Miglani)