Thai police fired stinging liquid from water cannon at thousands of Thai protesters on Friday in the most violent escalation of three months of demonstrations against the government of Prime Minister Prayuth Chan-ocha, a former junta leader.
Protesters, defying a government ban on gatherings for a second day, pushed back against helmeted police who advanced with batons and riot shields.
Youth-led protests have grown into the biggest challenge in years to a political establishment dominated by military figures and the Royal Palace of King Maha Vajiralongkorn.
"The dictatorial government is using violence to disperse the people's movement," said Tattep Ruangprapaikitseree, one of the protest leaders.
The king has made no direct comment on the protests, but in comments broadcast on state television, he said Thailand "needs people who love the country and love the monarchy".
Until now, police had not used major force to suppress peaceful protests that have drawn tens of thousands of people, although more than 40 demonstrators - including several leaders - have been arrested in the past week.
The government banned political gatherings of more than five people on Thursday. "We've issued warnings against illegal acts," police spokesman Yingyot Thepchamnong told reporters.
Reuters journalists said water turned on the protesters contained chemicals making them sting. Organisers told protesters to disperse more than three hours after they gathered.
"I am not afraid for myself. I fear more for the future of the country," said one protester Poom, 31, who did not want to give his full name.
Prayuth first took power as army chief in a 2014 coup. Critics say he engineered a general election last year to keep hold of power as a civilian prime minister.
He says the election was fair.
Protesters also want a new constitution, to replace one drafted under military rule.
"I'm not quitting," Prayuth told reporters after an emergency cabinet meeting, adding that emergency measures would be in force for up to 30 days.
Calls have also built up among protesters for reforms to the monarchy, which is accused by protesters of helping to entrench decades of military influence in politics.
The only specific incident cited by the government for the imposition of emergency measures was one in which Queen Suthida's motorcade was jeered, but it also said protests were damaging the economy and national security.
Police said on Friday that two men would be charged with attempted violence against the queen, which carries a possible death sentence if her life is thought to have been threatened. Even if not, the charge can mean life in jail.
The office of the United Nations' High Commissioner said it was concerned about the situation in Thailand, especially the application of serious charges, including sedition, against people peacefully exercising their rights.
Thai parliamentary opposition parties condemned the emergency measures.
"Pheu Thai Party calls on General Prayuth Chan-ocha and the state officials to lift the emergency decree and to stop intimidating the people in all manners and to release those who were arrested immediately," said the party, which has the most seats in parliament.