Bangkok-Thai police on Tuesday fired water cannon and tear gas on pro-democracy protesters attempting to get close to parliament, where lawmakers were debating possible changes to the military-scripted constitution.

Thailand has seen months of student-led demonstrations demanding reform to the constitution and monarchy, and for the current prime minister to quit.

Several thousand took to the streets around the parliament building Tuesday, where riot police had blocked off roads with concrete and barbed wire.

Police fired water cannon at protesters trying to dismantle a barricade, and later 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.

“What are police thinking by firing water cannon at people? The police are serving a dictator,” one protest leader said over a loudspeaker. 

At one point police were forced to retreat when protesters threw tear gas canisters back into their lines. Five people were taken to hospital for treatment, medical officials said.

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.