More than a thousand Thai pro-democracy protesters took over a major Bangkok intersection on Wednesday, defying authorities in the first major rally since police used rubber bullets against demonstrators over the weekend.

Thailand's protest movement kicked off in July, calling for the resignation of Prime Minister Prayut Chan-O-Cha's government and a rewrite of a military-scripted constitution.

But the most controversial demand is to reform the untouchable monarchy, including the abolition of the kingdom's draconian royal defamation laws.

So far more than 70 protesters and student leaders have been charged for allegedly insulting the monarchy and about a dozen are remanded in prison.

If convicted, they face up to 15 years in jail per charge.

On Wednesday, protesters gathered in Bangkok's main shopping district, chanting "release our friends" and "abolish 112" -- a reference to the lese majeste law in Thailand's criminal code that shields the monarchy from criticism.

A stage in the middle of the road had a huge banner that read "Monarchy reform" while scores of demonstrators held pictures of detainees and glued them onto street signs.

"It does not matter how many of our friends are arrested -- 10 or a hundred -- we will not stop coming out," Benja Apan, one of the protest leaders facing several royal defamation charges, told the crowd.

"It cannot stop our spirit. We will fight together. It will be good if our friends can be out fighting together with us."

A heavy police presence, including riot squads, was building in the vicinity in the early evening.

More than a thousand Thai pro-democracy protesters took over an intersection in Bangkok's main shopping district
More than a thousand Thai pro-democracy protesters took over an intersection in Bangkok's main shopping district AFP / Lillian SUWANRUMPHA

Ahead of the rally, National Police deputy spokesman Kissana Phathanacharoen warned the protest was illegal because of coronavirus restrictions and officers would "enforce the law step-by-step".

On Saturday, Thai police used water cannon and rubber bullets outside Bangkok's Grand Palace after demonstrators broke through a barricade of shipping containers.

Twenty protesters and 13 police officers were injured in the clash, according to a local emergency centre, while authorities said 20 people were arrested and seven have been charged with lese majeste.

Bangkok artist Chanaradee, 26 -- who came out Wednesday to the Ratchaprasong intersection -- said she wasn't put off by the police tactics.

Instead, she is angry about the "brutal" escalating use of force because her 70-year-old father was injured while running from rubber bullets.

"I'm very disappointed and sad. My dad went to the mob alone (on Saturday)... he hurt himself. He fell over," she told AFP.

She added that her father had been taking part in pro-democracy demonstrations since 1973, the year of a massive student uprising against a military dictator.

The fact "he still needs to do it (today) shows Thailand is not going anywhere," Chanaradee said, though she added that those in the pro-democracy movement today are able to talk about royal reforms.

"We are making progress. It's important we stay hopeful."