BANGKOK (AP) — Thailand’s political battleground shifted Tuesday from the streets to the country’s Parliament, where lawmakers are considering proposals to amend the constitution, one of the core demands of the nation's student-led pro-democracy movement.

Chances that sparring would be limited to the legislative chamber vanished in the afternoon, however, as pro-democracy protesters at the site were pushed back by police water cannons as they tried to breach barriers set up on several streets to stop them from penetrating the Parliament compound perimeter. Protesters said the water contained an irritant, which would not be unusual for crowd control.

As the initial small number of protesters increased later in the day to as many as 1,000, they achieved at least a tactical victory by removing some of the barbed wire-reinforced barriers in front of the main gate of the compound. They accomplished that even though police at one point fired tear gas canisters, some of which were tossed back by demonstrators.

Video broadcast from the scene showed a large number of people being helped or carried from the site after water cannons and tear gas were deployed, though there were no reports of serious injuries. Many of the protesters wore white construction helmets and other protective gear.

Several lawmakers left the scene by boat from a pier behind Parliament even as the debate inside was scheduled to continue for many more hours, until midnight.

Seven draft constitutional amendments are scheduled to be voted on in a two-day joint session of the House and Senate. Constitutional changes require a joint vote of those two bodies. Any motions that are passed will have to go through second and third readings at least a month after this week’s vote.

Thailand has had 20 constitutions since abolishing the absolute monarchy in 1932 in favor of a constitutional monarchy.

It is not expected that Parliament at this point will agree on specific changes for inclusion in a new charter, though the protesters back a draft that would roll back aspects of the current 2017 constitution — enacted during military rule — that granted extra powers to unelected branches of government, such as the Senate.

Instead, Parliament is likely to establish a constitution 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.

Consensus could also be reached on a draft that would allow all points in the constitution to be amended, with the significant exception of articles concerning the monarchy. Reform of the monarchy is another key demand of the protest movement, which believes the royal institution is too powerful and lacking accountability.

But any consideration of sections concerning the monarchy is fiercely opposed by the government and its supporters, who consider the institution untouchable. The sole draft that calls for considering amending all parts of the constitution is almost certain to be rejected.

The pro-democracy movement, which supports substantial changes to the constitution, said ahead of the meeting that it planned to have its followers surround the Parliament building in a show of strength. The movement has been staging increasingly determined mass rallies of thousands of people around the country for months.

A contending group of a couple of hundred royalists who oppose changes gathered outside of Parliament on Tuesday morning as the session opened and departed ahead of the afternoon arrival of the pro-democracy group. But several busloads more arrived in the afternoon, and the two sets of demonstrators at one point threw rocks and other objects at each other.

The parliamentary session is an effort by the government of Prime Minister Prayuth Chan-ocha to take the initiative away from the pro-democracy movement, which in addition to seeking constitutional changes and reforms to the monarchy wants Prayuth and his government to step down.

The protesters allege that Prayuth, who as army chief in 2014 led a coup that ousted an elected government, was returned to power unfairly in last year’s election because laws had been changed to favor a pro-military party. The protesters also say the constitution, written and enacted under military rule, is undemocratic.

It is their unprecedented demand for reforms to the monarchy that has caused the most stir. The issue touches a raw nerve in Thailand, where reverence for the royal institution is inculcated from birth and protected by a law that makes defaming the monarch and his immediate family punishable by up to 15 years in prison.

The most controversial amendment is the one proposed by a progressive civic association, the Internet Law Reform Dialogue, which collected about 100,000 signatures to put it on the parliamentary agenda. It seeks many specific changes to the 2017 charter, but its biggest sticking point is that it would allow changes to be made in articles in the constitution covering the monarchy.

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Associated Press writers Chalida Ekvitthayavechnukul and Grant Peck contributed to this report.