A 62-year-old Thai man who became known as "Uncle SMS" after he was convicted of defaming Thailand's royal family in text messages died Tuesday while serving his 20-year prison term.

The case of Amphon Tangnoppakul, a grandfather who had suffered from cancer, drew attention to Thailand's severe lese majeste law last November when he received one of the heaviest-ever sentences for someone accused of insulting the monarchy.

As news of his death spread, about 40 demonstrators gathered outside Bangkok's Central Criminal Court holding signs denouncing the defamation law and holding leaflets saying "Uncle is dead. Who killed him?"

His wife, Rosmalin Tangnoppakul, learned of his death while trying to visit him Tuesday at the Bangkok prison where he was being held. Friends later consoled her at a prison reception area while she burned and incense stick and prayed.

"Amphon Tangnoppakul, you can come home now," she said. "You're free now. Come home!"

The cause of Amphon's death early Tuesday was not immediately known, but he had complained of stomach pains on Friday and was transferred to a correctional department hospital, his lawyer Anon Numpa said. Officials planned an autopsy on Wednesday, the lawyer said.

Amphon was arrested in August 2010 and accused of using a mobile phone to send four text messages to a government official that were deemed offensive to the queen. He denied sending them, however, and said he didn't even know how to use the SMS function on his telephone to send texts.

He wept during his court proceedings, saying, "I love the King."

The sentence was believed to be the heaviest ever received in a lese majeste case because of additional penalties issued under a related law, the 2007 Computer Crimes Act.

Before his arrest, Amphon had lived with his wife, daughter-in-law and three grandchildren in a rented room in Samut Prakan province, on the outskirts of Bangkok. He was retired and suffered from cancer of the mouth.

Opponents of the lese majeste law -- known as Article 112 -- have been seeking its reform or abolition, but have met with fierce opposition by royalists.

"He was the victim of this draconian law, Article 112," said Suda Rangkupan, a Chulalongkorn University lecturer and member of the group Friends of Thai Political Prisoners. "And we also want to show to the court ... to the Thai court, that this death was caused by injustice in Thailand."