About 1,000 people gathered in a Bangkok sports arena on Sunday, swearing to defend Thailand’s monarchy from a student-led mass movement that they believe is a threat to the institution’s existence.

Calling themselves “Thai Pakdee,” or “Loyal Thai,” most were dressed in yellow shirts — a colour closely associated with the monarchy and the ruling establishment. Some clutched portraits of current King Maha Vajiralongkorn and of his late revered father, King Bhumibhol Adulyadej, while others waved giant national flags.

The gathering took place as anti-government protests gather momentum across Thailand, posing the strongest challenge yet to the rule of Prime Minister Prayuth Chan-ocha.

The former army chief first took power in a coup in 2014, then retained it in an election last year widely seen as rigged to all but guarantee his victory.

Speakers at Sunday’s rally pledged to protect the monarchy and the nation. Videos were shown, one of which extolled King Maha’s role in fighting communism in the 1970s, when a jungle insurgency threatened the country’s stability.

“I promise to fight shoulder to shoulder with brothers and sisters who share the same beliefs, to protect the beloved institution,” said one speaker, Patiyut Tongpajong.

Speaking to the media before the program began, Thai Pakdee leader Warong Dechgitvigrom, a former Member of parliament, said their aim was “to educate the people.” The protesters that the royalist movement is responding to have three core demands: holding new elections, amending the constitution and ending the intimidation of critics of the government.

But some of the anti-government movement’s leaders stirred controversy with an unprecedented call for reform of the monarchy, which they see as out of touch and unaccountable.

The institution has traditionally been treated as beyond reproach. It is also protected by strict defamation laws that can bring punishment of 15 years in prison.