Thousands of Thais joined protests that popped up across Bangkok on Saturday in defiance of a crackdown on three months of demonstrations aimed at the government and the powerful monarchy.

Police used water cannon for the first time on Friday and shut much of the city's transport system on Saturday to try to thwart protesters, but they gathered wherever they could.

Tang, a 27-year-old office worker said she joined thousands of people protesting at the Lat Phrao station on Saturday after seeing the pictures of police firing water cannon at youth-led protesters, including many school children.

Police officers line up during anti-government protests, in Bangkok.

"It was way over the line. We want to show them our power and that we can’t accept this," she said. Many other protesters voiced the same anger and said they were out for the first time.

Protests have drawn tens of thousands of people to the streets to demand the removal of Prime Minister Prayuth Chan-ocha, a former military ruler. They have also become openly critical of King Maha Vajiralongkorn despite lese majeste laws that can mean 15 years in jail for insulting the monarchy.

On Thursday, the government banned all political gatherings of five or more people. Police have arrested more than 50 people — including several protest leaders — in the past week.

"Violent or not, all gatherings are illegal," police spokesman Yingyos Thepjamnong told a news conference.

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But police made no immediate attempt to stop the protest at Lat Phrao.

Government spokesman Anucha Burapachaisri told Reuters: "There is no win or lose for any side. It's all damage to the country."

The Royal Palace has made no comment on the protests but the king has said Thailand needed people who love the country and the monarchy.

Protesters say Prayuth engineered last year's election to keep power he seized in a 2014 coup - an accusation he denies. They say the monarchy has helped perpetuate years of political influence by the army and seek to curb its powers.

The protesters at Lat Phrao chanted "Prayuth get out" and invective calling him "Tu", the premier's nickname.

"I condemn those who cracked down on the protesters and those who ordered it. You all have blood on your hands," protest leader Tattep Ruangprapaikitseree, said after being freed on bail following his arrest on Friday.

Pro-democracy demonstrators attend a protest in Bangkok, Thailand, on Saturday. Reuters

Bail was also granted to one of two activists charged with trying to harm the queen - rarely used charges that carry a potential life sentence — after protesters shouted at her motorcade on Wednesday.

Human rights groups have condemned the dozens of arrests and the use of force against peaceful protests.

"Concerned governments and the United Nations should speak out publicly to demand an immediate end to political repression by the Prayuth administration," said Brad Adams, Asia director at Human Rights Watch, in a statement.