Thai police on Tuesday fired water cannon and tear gas on pro-democracy protesters trying to reach parliament, where lawmakers were debating possible changes to the military-scripted constitution.
About 40 people were injured, including five who were shot, according to emergency services.
The wounded included one royalist supporter shot in the hip and a pro-democracy protester hit in the leg, police said.
Police used tear gas and water cannons laced with irritating chemicals against the student-led demonstrators, who tried to push their way past barbed-wire barriers to enter the grounds of the legislature on the outskirts of Bangkok.
Yet despite an ostensible return to democratic rule after parliamentary elections in March past year, Prayut's government "is showing no signs of loosening its grip" and continues to employ various forms of repression, the rights group added.
"Slaves of tyranny", the protesters chanted outside the building in central Bangkok.
In addition to the confrontation between police and protesters, there was also a fight between protesters and a group of royalists. The protest movement has been staging increasingly determined mass rallies of thousands of people around the country.
The clashes occurred as people gathered close to the Parliament as lawmakers were discussing changes to the constitution.
Any motions that are passed will have to go through a second and third reading at least a month after today's vote.
The proposed constitutional amendments come after months of protests. Student-led rallies have rocked Thailand for months, demanding a constitutional overhaul and the removal of Prime Minister Prayut Chan-O-Cha, who took power in a 2014 coup.
Some 20 groups called the rally at Bangkok's Democracy Monument under the name "Mob Fest" as the latest in a series of protests calling for significant reforms in government.
"We need to protect the monarch", senator Seri Suwanphanon said as he spoke against the iLaw proposal, which is backed by protesters and many opposition members of parliament.
Some in the movement have also called for reforms to the monarchy - a once-taboo subject - sending shockwaves through the Thai establishment. Thailand's lèse-majesté law, which forbids any insult to the monarchy, is among the strictest in the world.
Some protesters threw glass bottles and paint bombs over the walls of police headquarters, which was barricaded with dumper trucks, concrete blocks and razor wire.
The violence on Tuesday started when a group of protesters tried to cut through wire barricades surrounding the Thai parliament. No further details were available. Police have denied using rubber bullets or live rounds. Some lawmakers had already left the scene by boat from a pier behind Parliament.
In parliament, where lawmakers voted on seven options for ways to change the constitution, a proposal that would have opened the way for discussion of the king's role failed to win enough votes for approval.
Protesters demand changes to the constitution drawn up by Thailand's former junta.
Protesters wearing multi-colored rain ponchos, goggles and hard hatsthrew smoke bombs and paint at police.
The Parliament's charter amendments likely won't address all of the demands from the protesters, according to political analysts. Reports said at least 55 people were injured in clashes as security officials fired teargas shells.
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