Amnesty International has called for the government to withdraw charges against anti-government protesters and asked its own supporters to send letters backing the demand to Prime Minister Prayut Chan-o-cha.
The charges laid against 31 protesters for the July 18 rally against the government were aimed at silencing critics and were a violation of freedom of expression, the London-based human rights advocacy group said in a statement issued on Friday.
The statement said the charges, which included sedition, should be immediately dropped.
Amnesty called on people around the world to send letters to the Thai prime minister supporting its demand.
"This recent slew of arrests of peaceful demonstrators illustrates the authorities' intensifying crackdown on freedom of expression and assembly," it said.
The pro-democracy rally at the Democracy Monument in Bangkok on July 18 was the first since the country was hit by the coronavirus pandemic early this year.
It attracted about 1,000 demonstrators. It was followed by a series of small rallies around the country, and a much larger rally at Democracy Monument on Aug 16.
The protesters are calling for a new constitution, an end to intimidation of critics of the government, and a House dissolution and general election.
Rights lawyer Arnon Nampa and university academic Panupong Jadnok were among the 31 people charged by police.
The two are being held in custody after the Criminal Court on Thursday revoked their bail at the request of Samran Rat police. The court accepted the police argument that the two men broke the court bail stipulation that they refrain from similar offences.
The charges carry a maximum penalty of seven years behind bars. The two activists are being detained in Bangkok Remand Prison. (continues below)
A rally was planned outside the prison on Friday to demand their release.
Protest leaders have set Sept 19 for another major rally, and promised there would be a much larger crowd at the Democracy Monument and in Ratchadamnoen Avenue that day.
Mr Anon posted a Facebook message on Thursday calling for people to go out on Sept 19. "Let my detention be a 'receipt' of intimidation against people. Sept 19, 2020 will be the day to respond," he wrote.
He called his detention a "gamble for change" and called for support for the rally in two weeks' time, to show that protesters were moving in the right direction.
