The protesters are calling for Prime Minister Prayuth Chan-ocha to leave office, the constitution to be amended to make it more democratic and the nation’s monarchy to undergo reform.
Thousands of Thai pro-democracy protesters have massed in multiple locations across Bangkok, defying an emergency decree banning gatherings for the third consecutive day after confrontations saw riot police use water cannon on peaceful demonstrators.
About 3,000 demonstrators in the city's main shopping mall district demanded the release of arrested protesters, and some shouted obscenities against Prime Minister Prayut Chan-ocha before they were dispersed by police.
Across the Chao Phraya river, around 2,500 rallied in the western Wongwian Yai district chanting "Long live the people, down with dictatorship!", while in southeastern Udomsuk another 5,000 brought busy traffic to a standstill.
The Free Youth, one of the movement's main organising groups, had warned protesters in an online post to be "prepared both physically and mentally... and to cope with a crackdown if it happens."
Operators of both the Skytrain and underground rail networks had shut down services city-wide to prevent protesters from joining.
Prime Minister Prayut Chan-ocha announced an emergency decree banning on Friday gatherings of more than four people would be imposed for at least a month.
The former army chief, who masterminded a coup in 2014 before being voted into power last year in an election protesters say was rigged in his favour, also rebuffed calls for his resignation.
Thai Lawyers for Human Rights told AFP that 65 activists had been arrested since Tuesday, including nine of the most prominent figures of the pro-democracy movement.
Earlier on Friday, two activists were arrested under a rarely used law banning violence against the queen because they were among a group surrounding a royal motorcade on We dnesday during a large demonstration.
Both men could face life in prison if convicted.
A Thai journalist was also briefly detained during Friday's protest, according to his online news outlet, Prachathai.
Premier Prayut has extended the emergency decree banning gatherings of more than four people until mid-November, ignoring activists' calls for him to resign.
The former army chief was the mastermind behind a 2014 coup before being voted into power in last year's election which protesters say was rigged in his favour.
Opposition party Pheu Thai has called on the government to lift emergency measures and free those detained.
Human rights groups condemned the government measures.
Human Rights Watch on Saturday warned the emergency decree gave police "the green light to commit rights abuses with impunity" and called for international condemnation.
"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.
"We condemn any violence against the people," the People's Movement said in a statement. "We will continue to protest on Oct. 17", it said, telling people to be ready for the police to use suppression tactics.
Government spokesman Anucha Burapachaisri told Reuters: "There is no win or lose for any side. It's all damage to the country. The government would like to ask protesters to not gather and remain peaceful."
On Thursday, it ordered a ban on protests which have become the biggest challenge in years to Prime Minister Prayuth Chan-ocha, a former junta leader, and have brought unprecedented criticism of King Maha Vajiralongkorn.
Immediately after the ban, tens of thousands of people protested in Bangkok in defiance. Thousands more rallied on Friday.
Police said that their response to Friday's protest had been proportionate and in line with international norms.
Three protesters and four police personnel were hurt on Friday, said the Erawan Medical Center, an emergency response unit in Bangkok.
Demanding curbs on power
Protesters demand the removal of Prayuth, who first took power in a 2014 coup. He rejects protesters' accusations that he engineered last year's election to keep power. Breaking a longstanding taboo, protesters have also called for curbs on the power of the monarchy.
The Royal Palace has made no comment on the protests but the king said Thailand needed people who love the country and the monarchy. His comments were broadcast on state television on Friday as police clashed with protesters in Bangkok.
The scenes of water cannon and protesters pushing back with umbrellas were similar to images from anti-government demonstrations in Hong Kong, with which Thai activists make common cause.
Hong Kong activist Joshua Wong tweeted a picture of himself giving the three-finger salute of Thai campaigners and holding up a #StandWithThailand hashtag.
"People should not be afraid of their governments," he said. "Only governments should be afraid of their people."