Pro-democracy protesters in Thailand battled police who sought to keep them from entering the grounds of Parliament.
The protesters were trying to press for constitutional changes as MPs debated whether to back proposed amendments.
About 40 people were injured, including five who were shot, according to emergency services. It was unclear who fired the shots and whether they were live rounds or rubber bullets.
Some of the injuries occurred during a brawl between the pro-democracy protesters and stone-throwing royalists who oppose constitutional change.
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.
The chaotic street confrontations began last night NZT and ended about six hours later, when protest leaders called a halt and sent followers home.
It was the worst violence during months of actions by the protesters, though police had previously employed water cannons at least twice. The protest movement has been staging increasingly determined mass rallies of thousands of people around the country.
Lawmakers are scheduled to vote on seven proposed constitutional amendments during a two-day joint session of the elected House and appointed Senate. Constitutional changes require a joint vote of both bodies. Any motions that are passed will have to go through second and third votes at least a month after this week's balloting.
MPs adjourned a previous session without voting on proposed amendments, leading the protesters to accuse the government of bad faith.
The parliamentary session is an effort by the Government of Prime Minister Prayuth Chan-ocha to take the initiative away from the pro-democracy movement, which in addition to seeking constitutional changes and reforms to the monarchy wants Prayuth and the Government to step down.
The protesters say that Prayuth, who as army chief in 2014 led a coup that ousted an elected government, returned to power unfairly in last year's election because laws had been changed to favour a pro-military party.
The protesters also say the constitution, written and enacted under military rule, is undemocratic.
Reform of the monarchy is a key demand of the protest movement, which believes the royal institution is too powerful and lacks accountability.
The unprecedented demand has touched a raw nerve in Thailand, where reverence for the royal institution is inculcated from birth and protected by a law that makes defaming the monarch and his immediate family punishable by up to 15 years in prison.
Parliament is not expected to agree on specific constitutional changes at this point.
Instead, Parliament is likely to establish a drafting committee to write a new charter.
This would allow the Government to say it is willing to meet the protesters' demands at least halfway, while buying time with a process that could extend over many months.