BANGKOK:Some students at Thailand's Thammasat University posed with cardboard cutouts of well-known critics of the monarchy on Saturday in a protest as King Maha Vajiralongkorn was to present degrees amid growing calls for royal reform.

Youth and student-led demonstrations that began in July by demanding the ousting of former junta leader Prayuth Chan-ocha as prime minister have increasingly called for curbs on the king's powers, breaking a longstanding taboo.

State broadcaster Thai PBS quoted a source as saying only around half of this year's 9,600 Thammasat graduates joined rehearsals that are essential for those attending the degree ceremony. Last year, it said, only 10 percent missed the final ceremony.

The Palace has made no comment since the protests began. Prayuth warned students on Friday not to disrupt the graduation ceremonies at Thammasat, long seen as a centre of radicalism.

Student protesters set up life-size cardboard cutouts of historian Somsak Jeamteerasakul and former diplomat Pavin Chachavalpongpun, widely followed critics of the monarchy who live in self-exile.

"I chose to take a picture with Somsak because I respect him and I don’t think he deserves whatever happened to him for speaking the truth and bravely criticising the monarchy openly," said one 23-year-old student, who gave his name as Marut.

Some students posed with a cardboard cutout of Bernard, a well-known nut seller on the campus for decades. Many students, at the ceremony with their families, ignored the cutouts.

The university did not respond to a request for comment.

The graduation ceremony with accompanying photos is a rite of passage in Thailand.

Many families display gilt-framed pictures of graduates receiving degrees from the monarch, a practice begun last century to strengthen royal bonds with the middle class.

This year's protests have challenged the prestige of the Palace to the greatest extent since the 1932 revolt that ended absolute monarchy.

A week long government crackdown in mid-October, with a ban on protests and the arrest of many protest leaders, backfired when it prompted bigger demonstrations.

Three of the best known protest leaders were re-arrested overnight after the expiry of the limit for keeping them detained. Pictures of one of them being carried unconscious from a police van fuelled protesters' anger.

Thai Lawyers for Human Rights said it believed Panupong "Mike Rayong" Jadnok, 24, had been put in a chokehold by police. Police spokesman Kissana Phatanacharoen told Reuters "he had a reaction and so was sent to the hospital," describing the arrest procedures as "normal".