One of Thailand's most prominent intellectuals has been arrested on a charge of insulting the country's monarchy, an offense that could send the 75-year-old to prison for 15 years, his lawyer said Friday.

Sulak Sivaraksa, 75, was arrested Thursday for remarks made during a speech he gave in December last year to mark International Human Rights Day, lawyer Somchai Homla-or said.

Sulak denied the charges and was freed on bail, the lawyer said. Insulting the monarchy, known as the crime of "lese majeste," carries a maximum penalty of 15 years in prison.

Sulak has been arrested several times before on lese majeste charges, but was never convicted, Somchai said.

The lawyer would not quote the passages from the Dec. 11, 2007 speech on philosophy, society and human rights that the police considered offensive. He said Sulak considered his critical remarks to be an effort to protect the monarchy.

Police recently said they are investigating about 30 such cases. A 41-year-old Australian writer is in jail awaiting formal charges for allegedly inappropriate passages in a novel.

In recent years, a nationalist fervor that emphasizes devotion to the monarchy has swept Thailand, making authorities more sensitive to possible criticism.

Thais in 2006 celebrated the 60th anniversary of King Bhumibol Adulyadej taking the throne. In the same year, a coup toppled Prime Minister Thaksin Shinawatra, who was accused of showing disrespect to the monarchy as well as corruption.

Thaksin's opponents continue to fight against his political machine, which they sometimes accuse of seeking to establish a republic. The government, led by Thaksin's allies, consistently avows its loyalty to the crown.

Almost all Thais revere the monarchy, and especially admire the 80-year-old king. However, the lese majeste charge is often used for political purposes as a way of smearing its targets.

The British-educated Sulak has been associated with reformist movements in Thailand since the 1960s, when he was the intellectual mentor to students who went on to take part in an uprising against a military dictatorship in 1973.

He fled abroad after a right-wing counterrevolution in 1976, the first of several periods he spent in exile.

A colorful figure who often wears traditional Thai clothing, Sulak is closely associated with Buddhist humanist causes, and is known to friends as a staunch monarchist.