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.
Eighteen people were injured in clashes as protesters inched closer to parliament, in the latest round of demonstrations demanding reform to the Thai constitution, monarchy, and for the current prime minister to quit.
As night fell, protesters and police were locked in a tense standoff at the main gates of the parliament building.
"Stop accusing us of violating the law, you (police) are violating the law by hurting people," prominent student leader Parit "Penguin" Chiwarak said through a megaphone from a truck.
"We are fighting for a better future of our country and for everyone so don't fire water cannon on us."
Earlier a police retreat at one barricade saw the first major clash between pro-democracy protesters and yellow-shirted ultra-royalist supporters, who threw bottles, rocks and rubbish at each other.
Several thousand democracy activists took to the streets around the parliament building in the afternoon, where riot police had blocked off roads with barriers and barbed wire.
Police fired water cannon at protesters trying to dismantle a barricade, and used water laced with irritant, sending demonstrators scrambling to wash their eyes.
Some sheltered behind giant inflatable rubber ducks which protesters had planned to float along the river behind parliament as lawmakers debated inside.
Student-led rallies have rocked Thailand since July, demanding the removal of Prime Minister Prayut Chan-O-Cha -- who took power in a 2014 coup -- and reform of the military-scripted constitution.
Some in the movement have also called for reforms to the monarchy -- a once-taboo subject -- sending shockwaves through the Thai establishment.
- Call for calm -
As police and protesters clashed outside, lawmakers were considering whether to debate seven possible constitutional amendments.
They include a proposal to replace the present military appointments in the Senate with directly elected representatives.
But any constitutional change in Thailand is expected to take a long time and the junta-appointed Senate is unlikely to vote itself out of power.
Several opposition lawmakers came out of parliament to monitor police tactics.
Taopipop Limjittrakorn, an MP from the pro-reform Move Forward Party said police were ignoring pleas to de-escalate the situation.
"Police should not be overly violent and threaten" protesters, he told reporters.
The confrontation between ultra-royalists and democracy activists looked like it would come to a peaceful end when a handful of people on both sides stopped to hug each other, but the bonhomie was short lived.
The royalists, who were permitted to rally in the parliamentary zone, are against all proposals to change Thailand's 2017 constitution, saying they would undermine the monarchy.
Parliament is expected to vote on Wednesday on which constitutional amendments bills will be debated.