Police in Thailand have used water cannon against protesters in central Bangkok, as the pro-democracy activists defied an emergency decree banning gatherings for a second night running.

Around 2,000 demonstrators in the city's main shopping mall district had been chanting for the release of arrested activists and hurled obscenities at Prime Minister Prayut Chan-o-cha.

Several hundred riot police advanced towards them in formation, calling on the protesters to go home or face the water cannon.

A few metres away, hundreds of activists blocked the road behind a makeshift barrier, calling for the riot police to "get out!" and singing the Thai national anthem.

Four officers and one protester were injured, according to a city hospital.

Police fired chemical-laced water from the cannon, pushing back the protesters who used umbrellas against the blue liquid.

The demonstrators raised their arms to display a three-fingered salute adopted from the "Hunger Games" movies.

Police later dispersed the protesters, but many vowed to return to the streets.

"The (use of force) will increase the numbers of protesters," said 21-year-old Nine, an engineering student. "Anger has built up inside."

The kingdom's political elite has been jolted by the youth-led movement that is demanding the government's resignation and issuing a once-taboo call for reforms to Thailand's powerful monarchy.

Yesterday, the prime minister imposed an emergency decree banning gatherings of more than four people, but about 10,000 ignored the measure, demonstrating late into the night.

Vowing to stay on, Mr Prayut warned protesters "not to violate the law" and police shut down the planned rally site, but activists changed location to the shopping mall district of Pathumwan.

One protester was arrested during today's rally, which follows the earlier detention of around two dozen of the most prominent activists, including two under a rarely-used law banning violence against the queen.

Ekachai Hongkangwan and Bunkueanun Paothong were among activists who crowded around a royal motorcade carrying Queen Suthida on Wednesday during a large demonstration near the capital's government house.

In a Facebook broadcast this morning, Mr Bunkueanun denied trying to harm the queen, insisting: "I am innocent. That was not my intention."

Both men could face life in prison under a law that has not been used for decades and punishes any "act of violence against the queen or her liberty".

It is not clear why the pair were singled out.

This is the first time such a serious charge has been levelled against pro-democracy activists, many of whom have already been hit with lesser charges, including sedition and breaking coronavirus rules on gatherings.

Their movement's demands include the abolition of a strict royal defamation law - which shields the monarchy from criticism - and for the royal family to stay out of politics.

Among the other top activists arrested was Panusaya "Rung" Sithijirawattanakul, whose detention was live-streamed on Facebook.

Anon Numpa, another leading activist, said he was forcibly taken by helicopter to Chiang Mai in northern Thailand.

His lawyer Krisadang Nutcharut said that Mr Anon had been refused bail and was being held in Chiang Mai prison.

"Keep on fighting! My freedom is a very small issue compared to the entire struggle for democracy," he posted on Facebook.

The opposition Pheu Thai party also called on the government to lift emergency measures, stop the harassment of protesters and free those detained.

Thailand's modern political history is dotted with periods of violent civil unrest and more than a dozen military coups, the most recent of which brought Mr Prayut to power in 2014.