Pro-democracy protesters – many wearing white construction helmets and other protective gear – were pushed back by police water cannons and tear gas as they tried to breach barriers outside the parliament compound on Tuesday.
Video broadcast from the scene showed a large number of people being helped or carried from the site. However, there were no reports of serious injuries.
Demonstrators also clashed with a contending group of royalist protesters, at one point throwing rocks and other objects at each other.
Several politicians left by boat from a pier behind parliament even though the debate was scheduled to continue until midnight.
Seven draft constitutional amendments are scheduled to be voted on in a two-day joint session of the House and Senate. Constitutional changes require a joint vote of those two bodies. Any motions that are passed will have to go through second and third readings at least a month after this week’s vote.
Thailand has had 20 constitutions since abolishing the absolute monarchy in 1932 in favour of a constitutional monarchy.
It is not expected that parliament at this point will agree on specific changes for inclusion in a new charter. However, the protesters back a draft that would roll back aspects of the current 2017 constitution – enacted during military rule – that granted extra powers to unelected branches of government, such as the Senate.
Instead, parliament is likely to establish a constitution 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.
Consensus could also be reached on a draft that would allow all points in the constitution to be amended, with the significant exception of articles concerning the monarchy. Reform of the monarchy is another key demand of the protest movement, which believes the royal institution is too powerful and lacking accountability.
But any consideration of sections concerning the monarchy is fiercely opposed by the government and its supporters, who consider the institution untouchable. The sole draft that calls for considering amending all parts of the constitution is almost certain to be rejected.
The pro-democracy movement, which supports substantial changes to the constitution, said ahead of the meeting that it planned to have its followers surround the parliament building in a show of strength. The movement has been staging increasingly determined mass rallies of thousands of people around the country for months.
The parliamentary session is an effort by prime minister Prayuth Chan-ocha to take the initiative away from the pro-democracy movement, which has also called for him to step down.
Protesters allege that Mr Prayuth, who as army chief in 2014 led a coup that ousted an elected government, was returned to power unfairly in last year’s election because laws had been changed to favour a pro-military party.
They also say the constitution, written and enacted under military rule, is undemocratic.
The most controversial amendment is the one proposed by the Internet Law Reform Dialogue, a progressive civic association, which collected about 100,000 signatures to put it on the parliamentary agenda. It seeks many specific changes to the 2017 charter, including changes to the articles covering the monarchy.
The issue 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.