Thousands of anti-government demonstrators gathered at the Thai capital's Democracy Monument on Wednesday to demand the resignation of ex-army chief turned prime minister, Prayuth Chan-ocha.

"Down with dictatorship. Long live democracy," the protesters chanted, as they set off on a planned march towards Government House, the prime minister's office.

The youth-led movement has been staging rallies since July to push for an overhaul of the government, a new constitution and an end to harassment of government critics. Some have also made controversial calls for reforms to the monarchy, long considered a sacrosanct part of Thai identity.

Several hundred counter-demonstrators, decked out in the royal color yellow, staged a rally of their own nearby to show support for King Maha Vajiralongkorn. A motorcade carrying the king and queen to a royal ceremony was due to pass by later in the afternoon.

Read more: Opinion: Thailand's democratic dictatorship

Activists urge calm and respect

More than 14,000 police were deployed ahead of the marches, which are taking place on the anniversary of a student uprising on October 14, 1973 that led to the toppling of a military dictatorship. 

Prominent activist Anon Numpa urged the crowd to remain calm and respectful: "There will be provocations from the other side so please trust in me — we don't want to clash with anyone," he said. "When the royal motorcade arrives, don't utter swear words."

Wednesday's demonstration comes a day after 21 pro-reform activists were arrested during clashes with police at the same site. Officials said they were "uncooperative" when authorities tried to clear the area for the king's motorcade. In a rare challenge to the monarch, protesters chanted and gave the movement's trademark three-finger salute as his car drove by. 

Read more: The struggle continues for Thailand's opposition

Royal reforms are taboo

Last month, tens of thousands of pro-democracy protesters turned out in Bangkok in one of the country's largest rallies in years. Activists argue Prime Minister Prayuth, who toppled an elected government in a 2014 coup, manipulated last year's general elections to ensure the military stayed in control — a charge he denies.

The protesters are also calling for curbs on the constitutional powers of the king and for him to transfer back the control he took of some army units. However, these public demands have been met with a backlash from the royalist establishment.

Absolute royal rule ended in a 1932 revolution when Thailand became a constitutional monarchy, but critics fear the country is sliding back in that direction under the current King Vajiralongkorn, who took over the throne from his father in 2016.

Under Thailand's royal defamation law, criticism of the monarchy is punishable with up to 15 years in prison.

nm/aw (Reuters, AP, AFP)