A Swiss man was jailed for 10 years Thursday for insulting Thailand's revered king by vandalising his portraits during a drunken spree.

Oliver Jufer, 57, had pleaded guilty to five counts of lese majeste -- the crime of offending the dignity of a sovereign -- for defacing several portraits of King Bhumibol Adulyadej with spray paint in the northern city of Chiang Mai.

He had faced up to 75 years in prison, but the court sentenced him to 20 years and then halved the term because Jufer had confessed.

"The court has punished him for insulting the king. This is a serious crime," judge Pitsanu Tanbuakli said.

"Because he confessed, the court has reduced his sentence to 10 years."

Jufer can appeal the ruling, but his court-appointed lawyer did not attend the sentencing. Jufer said nothing as he entered or left the court.

Prosecutors declined to comment on the case due to the sensitivities of speaking about Thailand's king.

In Geneva, a foreign ministry spokesman said the Swiss government would not seek Jufer's release, saying the jail term had been applied according to Thai law.

"The Swiss government does not intervene with authorities in cases where the procedure followed conformed to basic judicial principles," spokesman Jean Philippe Jeannerat told AFP.

Jufer is from Zurich but has lived mainly in Thailand for the last 10 years and has married a Thai woman, according to Thai authorities.

Security cameras videotaped him defacing the king's portraits on December 5, which is the king's birthday and a time of national celebration.

Thailand has been swept up in royal fever since the king's 60th anniversary on the throne in June last year.

The palace also became more prominent in Thai political life with a military coup in September, which was conducted with the king's apparent blessing.

The generals who staged the coup have repeatedly said that one of the reasons for ousting elected prime minister Thaksin Shinawatra was because he had been "impolite" to the monarch.

Prosecutors are investigating three claims of lese majeste against Thaksin. A fourth charge, accusing Thaksin of praying inside a temple reserved for royals, has already been dropped.

Prison sentences for lese majeste are unusual. Those convicted are frequently given royal pardons, but analysts said that with Thaksin facing similar charges, authorities could not go easy on Jufer.

"With the charges against Thaksin, they cannot be seen to be lenient on lese majeste," political analyst Thitinan Pongsudhirak said.

"At the same time, (Jufer's) case is an aberration," he added, explaining that acts of vandalism against the king are rare and would deeply offend most Thais.

Thailand's king is the world's longest-reigning monarch, and one of the few who is still protected by tough laws that prohibit any insult to the royal family.

Thai law allows anyone to file a lese majeste complaint with the police, which makes people reluctant to engage in any sort of public conversation about the king or his family.

Portraits of the monarch, who will be 80 this December, hang in every public building and shrines to him dot the sidewalks in major cities.

Although the trial has attracted international attention, Thai media have hardly mentioned it due to the difficulty of reporting about the king without committing lese majeste in the process.

However, the king himself has indicated that he is willing to allow greater public discussion and even criticism.

"I can be criticised that sometimes I might be wrong, so that I will know I am wrong," he said in a speech for his birthday in 2005.

Nonetheless, the nation's censors routinely block websites that contain foreign news reports or online discussions about the monarchy. Foreign books and movies about the king are also banned here.