Hundreds of Thai protesters gathered in defiance of a sweeping crackdown Thursday after authorities moved to crush months of pro-democracy demonstrations by imposing emergency powers and rounding up leading activists.

Protesters chanted "Prayut get out!" and "Free our friends!" as they confronted police at Ratchaprasong, a busy junction in central Bangkok, despite a new decree banning groups of more than four people.

Student leaders had earlier taken to social media to urge supporters to take to the streets. "Come out in force -- only giving moral support from home is not enough," said the Free Youth Movement, which had organised massive demonstrations in recent months.

The government of Prime Minister Prayut Chan-O-Cha, a former army chief who initially took power in a 2014 coup, has been the target of mounting, student-led protests which are also taking aim at Thailand's unassailable monarchy.

After the emergency measures were announced early Thursday, riot police dispersed hundreds of protesters who camped overnight outside the prime minister's office.

'Violation of my rights'

Three top activists were among nearly two dozen arrested, including Parit "Penguin" Chiwarak, according to Panusaya Sithijirawattanakul -- another prominent figure, whose own arrest was live-streamed on Facebook.

Anon Numpa, another leading activist, said he was forcibly taken by helicopter to Chiang Mai in northern Thailand "without my lawyer". "This is a violation of my rights and is extremely dangerous to me," he wrote on Facebook.

It was not immediately clear how those arrested were accessing their social media accounts. On Wednesday, there were unprecedented scenes as protesters crowded around the royal motorcade carrying Queen Suthida and Prince Dipangkorn, raising the three-fingered gesture of defiance adopted from "The Hunger Games" books and films.

"In the past when the royals drive by, we cannot even walk around the area. We have to stop everything and kneel on the ground," a protester told AFP. "I am so surprised. It is happening now, we are changing a lot and it has moved forward. We are breaking taboos."

The emergency measures also allow the seizure of "electronic communications equipment, data, and weapons suspected to cause the emergency situation", a government spokesman said in a statement. "These are orders banning gatherings of five or more people... and banning distributing of news through electronic media that can affect national security," the spokesman said.

The order was imposed after thousands of demonstrators rallied Wednesday around Bangkok's Democracy Monument ahead of a scheduled drive-by of the royal motorcade. While police cordoned most of the protesters away from the royal route, dozens were still present as the convoy passed.

Queen Suthida, sitting next to Prince Dipangkorn Rasmijoti, could be seen staring from a limousine as protesters flashed the three-fingered salute. Such overt challenges to the monarchy are unheard-of in Thailand, where the royal family's influence permeates every aspect of society.

Leading opposition figure Thanathorn Juangroongruangkit decried the crackdown, calling for the government to "free all arrested people". "The government must quickly find a way to respond to protesters' demands, otherwise the situation will fan out nationwide," he said.

Turbulent history

The king spends much of his time in Europe, but has been in Thailand in recent days for an annual Buddhist ceremony and the anniversary of his father's death. Enormously wealthy, he is supported by the powerful military -- which has long positioned itself as the defender of the monarchy -- as well as the establishment elite.

There have been several popular uprisings in the turbulent modern history of Thailand, which has endured long stretches of political unrest and more than a dozen military coups since 1932. In the latest protests, leaders have repeatedly said they wish only for the monarchy to adapt to modern times.

Their demands include the abolition of a strict royal defamation law -- which shields the king from criticism -- and for the monarch to stay out of politics. Since these protests started, dozens of activists have been arrested, charged with sedition, and released on bail.

Government spokesman Anucha Burapachaisri said the premier had ordered police to press charges against protesters who obstructed the royal motorcade, and "those who had acted in a way that defames the monarchy".

"They must face legal procedures without exception."