BANGKOK • Thailand's Parliament rejected a push by pro-democracy groups for a Constitutional amendment to reduce the powers of the nation's monarchy, potentially escalating months-long anti-government protests.

The lawmakers instead voted for setting up a charter rewriting committee that will skip any review of chapters governing the monarchy, the most powerful institution in Thailand.

The roadmap for amendment proposed by a rights group, which the protesters support, failed to secure enough support from the lawmakers.

With the Parliament turning down the demand for monarchy reform, protest groups have vowed to stage mass gatherings until all of their demands are met, which also include the resignation of Prime Minister Prayut Chan-o-cha and a rewriting of the charter to make it more democratic.

The Parliament decision came as thousands of anti-government activists held a rally for a second straight day in the capital, after the most violent protest in years a day earlier left more than 50 people injured. They rallied to the police headquarters in central Bangkok yesterday to protest against the use of water cannons and teargas on demonstrators.

"The vote shows that most members of Parliament chose to uphold the power of feudal dictatorship and ignore the demands of the people," Free Youth, one of the protest groups, said in a statement.

"It can be predicted that the new Constitution to be drawn up won't reflect the will of the people because monarchy reform won't be part of the discussion by the rewriting committee."

Thailand's current charter has been the point of contention since its inception after Mr Prayut, a former army chief, took power in a 2014 coup.

Protesters and critics viewed it as instrumental in helping the royalist establishment retain its grip on power with Mr Prayut returning as the leader after last year's elections.

The charter allows the military-appointed Senate, whose powers protesters want scrapped, to vote for the premier.

Thai protesters have broken long-held taboos about publicly criticising the royal family and questioning laws that stifle discussion of the monarchy.

Free Youth has said the charter should be amended to "bring the monarchy under the Constitution".

The Parliament's charter amendments likely will not address all of the demands from the protesters, according to political analysts.

The vote yesterday is the first of the three planned that could pave a way to amend the charter, a process that is expected to take about a year or longer.

On Tuesday, a rally near Parliament turned violent after the police fired water cannons mixed with irritants to disperse the crowd and some pro-government supporters clashed with the protesters.

The clashes point to growing political unrest with pro-establishment groups also staging regular demonstrations. At least six people were injured from gun shots during Tuesday's protest, according to emergency medical services, but police denied using rubber bullets or live ammunition.

Mr Prayut, who has repeatedly rejected calls to quit, said yesterday he has asked security officials to ensure the safety of all sides during the protests, and urged activists to respect the law and avoid violence.

Bangkok police deputy commissioner Piya Tavichai said officers are investigating the shootings, which happened about 300m from the main protest zone.

He said one royalist supporter was shot in the hip and a pro-democracy protester in the leg.

"The rest of shooting victims are still under investigation, but the fact was both sides were shot and wounded," he said yesterday.

The Thai Human Rights Lawyers Association slammed police tactics, saying they were "not in accordance with international procedure to disperse demonstrations".

BLOOMBERG, AGENCE FRANCE-PRESSE