Thailand’s government has imposed a state of emergency.

Thailand’s government has imposed a state of emergency in a bid to end three months of student-led street protests calling for reforms to the monarchy and the resignation of Prime Minister Prayuth Chan-ocha, arresting at least 20 activists and two of the movement’s leaders early on Thursday.

The ruling bans gatherings of five or more people and the publication of news or online messages that could harm national security.

Protests have escalated over the past three months and on Wednesday tens of thousands of people marched in Bangkok, the capital, setting up camp outside Government House, the prime minister’s office. The government said it also acted after demonstrators obstructed a royal motorcade.

Videos shared widely on social media showed police protecting the royals’ yellow car as it moved through crowds of people holding their arms aloft in the three-finger salute that has become the symbol of the democracy movement and shouting their demands.

“It is extremely necessary to introduce an urgent measure to end this situation effectively and promptly to maintain peace and order,” state television said.

The announcement was accompanied by a document setting out measures that took effect from 4am local time (21:00 GMT) banning large gatherings and allowing authorities to ban people from entering any area they designate.

It also prohibits: “publication of news, other media, and electronic information that contains messages that could create fear or intentionally distort information, creating misunderstanding that will affect national security or peace and order.”

Shortly afterwards, police cleared the remaining protesters from outside Government House. Police said they had arrested protest leaders Parit “Penguin” Chirawat and rights lawyer Arnon Nampa. Thai Lawyers for Human Rights said its earlier statement that Panupong Jadnok had also been arrested was incorrect.

A third leader, Panusaya “Rung” Sithijirawattanakul, was picked up later on Thursday with pictures on social media showing her being taken away in a wheelchair as she gave the three-fingered salute. Rung had said a protest would take place at 4pm (09:00 GMT) despite the emergency decree. Police made no immediate comment.

The Asia desk of FIDH, an international human rights group, said at least 20 pro-democracy activists had been arrested. Under the state of emergency, police can detain people without charge for as long as 30 days.

“The scale of today’s arrests seems completely unjustified based in yesterday’s events,” Amnesty International’s Deputy Regional Director for Campaigns, Ming Yu Hah said in a statement, urging the authorities to release the detainees. “The assemblies were completely peaceful. These moves are clearly designed to stamp out dissent, and sow fear in anyone who sympathises with the protesters’ views.”