* Royal supporters also turn out in their thousands
Thousands of Thai protesters set up camp outside Prime Minister Prayuth Chan-ocha's office late on Wednesday, in an escalation of three months of demonstrations aimed at forcing the former junta leader to step down.
The protesters had earlier faced off against thousands of royalist supporters, who showed up in their biggest numbers since the start of protests that have also demanded curbs on the monarch's powers.
"We won't leave until Prayuth resigns," protest leader Arnon Nampa told the crowd gathered at the gates of Government House, many sitting on mats on the ground. The protesters said they planned to stay for at least three days.
Police spokesman Kissana Phathanacharoen said the presence of protesters within 50 metres of Government House was illegal. But government spokesman Anucha Burapachaisri told Reuters police had been told to avoid needless confrontation.
Officials at the premier's office were told not to come to work on Thursday.
The protest movement, led mainly by students and young people, aims to remove Prayuth, who took power in a 2014 coup that was meant to end a decade of violence between supporters and opponents of the country's establishment.
Those marching on the streets also want a new constitution and have called for the king's powers to be reduced.
At Government House, some protesters chanted insults at imposing gold-framed pictures of King Maha Vajiralongkorn - an act potentially punishable by up to 15 years in prison under strict lese majeste laws.
Tens of thousands of protesters marched to Government House, Reuters journalists estimated. Protesters put the number of more than 100,000 and said it was their biggest rally yet. Police put the number at 8,000.
They had gathered earlier at Bangkok's Democracy Monument, where they faced off with yellow-shirted supporters of the royal family hoping to catch a rare glimpse of the monarch. He spends much of his time in Germany.
Municipal trucks brought hundreds of workers to join the pro-monarchy crowd. One man among them appeared to sympathise with the protests, giving the three-fingered salute that has become the symbol of resistance against the political establishment. Demonstrators rushed to shake his hand.
Most royalists dispersed quickly after the motorcade carrying the king had passed, but some protesters later slowed a convoy carrying Queen Suthida, giving the three-finger salute and chanted "get out" at police protecting the vehicle.
Video on social media showed the queen smiling through the car window.
One royalist leader, Buddha Issara, said the protesters could demand democracy but must not call for reforms of the monarchy, as some have done.
"They must not touch the institution," he told reporters.
Protesters made a rare direct challenge to the king on Tuesday, chanting at his passing convoy after 21 activists were arrested during scuffles with police. The detainees were charged with public order offences on Wednesday.
The royal palace has not responded to requests for comment on the protests or the protesters' demands.
Among the anti-government protesters' demands are for curbs on the constitutional powers of the king and for him to transfer back personal control he took of some army units and a palace fortune valued in the tens of billions of dollars.
They say Prayuth manipulated elections last year to ensure the army's continued hold over the establishment. He says the elections were fair.
"Things shouldn't be like this," said one 17-year-old student at the protest who gave her name only as Foil. "We have to take back our future."
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