BANGKOK (AP) — Police in Thailand's capital braced for possible trouble Wednesday, a day after a protest outside Parliament by pro-democracy demonstrators was marred by violence that left 55 people injured, including as many as six with gunshot wounds.

The protesters, enraged at what they regard as disproportionate use of force against them by police who employed chemical-laced water cannons and tear gas, gathered for another protest late Wednesday in central Bangkok.

Tuesday's protest was held as a joint session of the House and Senate debated seven motions for amending the constitution. A vote on the motions was taking place Wednesday. Any that are passed will have to go through second and third votes after at least a month.

According to Erawan emergency services, 55 people at Tuesday’s protest were sent to hospitals, the largest number suffering from the effects of tear gas. It said four remained hospitalized Wednesday, including three of the six people it said had suffered gunshot wounds. The circumstances of the shootings were not announced.

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

It was the worst violence during months of actions by the pro-democracy protesters, though police had previously employed water cannons at least twice. The protest movement has been staging increasingly determined mass rallies of thousands of people around the country.

Police at a news conference Wednesday spoke of only two people with gunshot wounds, and indicated that they were not responsible for them.

“We are in the process of an investigation into who might be behind the shootings,” said police deputy spokesperson Col. Kissana Phathanacharoen. “No rubber or live bullets were used by the police for yesterday’s police operation, that is confirmed.”

The planned venue of Wednesday's protest, the Rajprasong intersection in the middle of the city’s upmarket shopping district, is just minutes away from the national police headquarters. Groups of police drilling and riot control vehicles could be seen in its courtyard Wednesday morning.

Tuesday's protest was held to pressure lawmakers to approve the most comprehensive of the seven motions on amending the constitution. The motion, submitted by iLaw, the Internet Law Reform Dialogue, would allow all aspects of the charter to be changed, including articles dealing with the monarchy. It is almost certain to be voted down.

To Thailand’s royalist establishment, the monarchy is a untouchable institution that is the heart and soul of the nation. Public criticism of it is unprecedented, especially since a lese majeste law makes defaming the monarch and his immediate family punishable by up to 15 years' imprisonment.

The protesters believe the monarchy holds too much power under what is supposed to be a constitutional democracy, and want to make the royal institution more transparent and accountable. They also want Prime Minister Prayuth Chan-ocha and his government to step down and for the constitution to be amended to make it more democratic, with less power given to unelected bodies such as the Senate.

Most of Tuesday’s violence occurred when police used tear gas and water cannons laced with irritating chemicals against the student-led demonstrators as they tried to push their way past barbed-wire and other barriers to enter the grounds of the legislature on the outskirts of Bangkok.

After about six hours of chaos, a protest leader, Parit “Penguin” Chiwarak, announced the end of the protest, saying the demonstrators had captured enough ground to declare they had achieved their goal of surrounding Parliament.