Thai riot police blasted water cannon on protesters who tried to cut their way through razor wire barricades outside parliament on Tuesday, as lawmakers discussed possible changes to the Thai constitution.
It was drawn up by Thailand's former junta, and protesters want amendments.
They also want the removal of Prime Minister Prayuth Chan-ocha, a former army ruler, and reforms to curb the powers of the monarchy.
Police set up the barricades and banned protests within 50 meters of the area.
They also used tear gas on pro-democracy protesters, who responded by throwing colored smoke bombs at them.
Several people were injured.
Lawmakers inside parliament were discussing several proposals for amending the constitution, some of which left the King's powers untouched.
Protests, which picked up in July, initially targeted Prayuth and constitutional change.
But they've since evolved to add calls for the monarch's role to be more accountable, and for Thailand to scrap changes that gave the current king personal control of the royal fortune and some army units.
The opposition represents a dramatic change in a country where, just months ago, any criticism of the monarchy was taboo - since the institution must be revered under the constitution.
Hundreds of royalists earlier gathered to call on lawmakers not to change the constitution.
They suspect the anti-government protesters want to abolish the monarchy altogether, which they deny.