At least 55 people have been injured, some with gunshot wounds, when demonstrators marching on the Thai parliament clashed with police and royalist counter-protesters, in the worst violence since a new youth-led protest movement emerged in July.

Police fired water cannon and tear gas at protesters who on Tuesday cut through razor-wire barricades and removed concrete barriers outside the parliament.

The police denied that they had opened fire with live ammunition or rubber-coated bullets and said they were investigating who might have used firearms.

The protest movement, which has called for deep constitutional reform to a system demonstrators say has entrenched the power of the military, has emerged as the biggest challenge to Thailand’s establishment in years.

Thousands of demonstrators converged on parliament to put pressure on legislators discussing changes to the constitution. The protesters also want the removal of Prime Minister Prayuth Chan-ocha, a former army ruler, and to curb the powers of King Maha Vajiralongkorn.

Bangkok’s Erawan Medical Centre said at least 55 people were hurt. It said at least 32 were suffering from tear gas and six people had gunshot wounds. It did not say who might have used firearms.

“We tried to avoid clashes,” the deputy head of Bangkok police, Piya Tavichai, told a news conference. He said police had tried to push back protesters from parliament and to separate them and the yellow-shirted royalist counter-protesters. ‘There will be no compromise’

During the street confrontation, protesters advanced on police with makeshift shields, including inflatable pool ducks. After about six hours, police pulled back and abandoned their water trucks, which the protesters mounted and sprayed with graffiti.

“I hereby announce the escalation of the protests. We will not give in. There will be no compromise,” Parit “Penguin” Chiwarak told the crowd at the gates of parliament before protesters dispersed.

Another protest was set for central Bangkok on Wednesday.

Government spokesman Anucha Burapachaisri said police had been obliged to use tear gas and water cannon to keep parliamentarians safe.

As police and protesters clashed outside, legislators 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.

Parliament is expected to vote on Wednesday on which constitutional amendments bills will be debated.

Protests that picked up in July initially took aim at Prayuth and constitutional change but have since called for the monarch’s role to be more clearly accountable, and for the reversal of changes that gave the king personal control of the royal fortune and several army units.

Prayuth led the 2014 coup that overthrew the democratically elected government.

Before the anti-government protesters reached Parliament on Tuesday, several hundred royalists dressed in yellow, the colour representing the monarchy, gathered there to urge legislators not to make changes to the constitution.

Some of the injuries occurred during a brawl between the pro-democracy protesters and stone-throwing royalists.

Al Jazeera’s Scott Heidler, reporting from Bangkok, said there were concerns of these “two sets of protesters seeing each other eye to eye”.

“There was a clash … a sustained clash for about 10, maybe 15 minutes,” he said. “Nothing major but that’s the first time we’ve seen this.”

Constitutional changes require a joint vote of the elected House and the appointed Senate. Any motions that are passed will have to go through second and third votes at least a month after this week’s balloting.

Parliament is not expected to agree on specific constitutional changes at this point. Instead, it 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.9