BANGKOK (AP) — Thai activists hoping to keep up the momentum in their campaign for democratic change held a third major rally in Bangkok on Wednesday, amid concerns about a possible confrontation with police or rival groups supporting the government.

Despite a massive security presence and harassment from counter-demonstrators, thousands of protesters marched from Bangkok’s Democracy Monument toward Government House, the offices of Prime Minister Prayuth Chan-ocha. “Prime minister, get out!” they chanted.

Before leaving Democracy Monument, several small clashes broke out between protesters and their opponents, who traded punches and threw plastic bottles as police tried to keep them apart.

There was speculation that the counter-protesters were organized by the authorities, with videos on social media showing municipal trucks carrying groups to the site.

The protesters negotiated or pushed their way without much resistance past several police roadblocks before reaching their target, the streets outside Government House, after almost four hours. Protest leaders announced plans to stay there for at least three days. Deputy police spokesman Col. Kissana Phathanacharoen estimated the crowd at 8,000.

The protest got off to a rocky start after organizers issued a post-midnight call for followers to begin assembling at Democracy Monument at 8 a.m. to assure they could secure the venue for the rally’s scheduled 2 p.m. start.

The area was blanketed with police, stationed in an organized manner but wearing yellow sports shirts instead of standard uniforms. Yellow shirts are a symbol of devotion to the monarchy, and are strongly associated with conservative politics.

The situation had already been complicated by King Maha Vajiralongkorn’s plans, which included a scheduled drive past the protest venue to attend a royal ceremony. The protesters said they would make way but there was a possibility that they could at a minimum show public disrespect for the crown. Several cars normally used by the royal family later were seen on nearby streets but their occupants could not be confirmed. Unverified video and photos on social media showed what was purported to be protesters gesturing and shouting close to the vehicles, which would be unprecedented for Thailand, where the royal family has traditionally been revered.

The king made a similar drive past the area on Tuesday after police cleared tents set up near the monument and arrested 21 people on minor charges.

Wednesday’s protest was held on the anniversary of a key date in a 1973 popular uprising that led to the toppling of a military dictatorship.

Historically, attempts to bring democratic reform to Thailand have eventually been reversed by military coups.

“We have to fight,” said university student Kanokwan Kawkaew, 20. “If we don’t fight, we will lose again.”

The crowds at the previous two rallies, held on weekends, were significant, with an estimated 20,000 people attending a Sept. 19 event. But turnout appeared to be lower for this rally, launched on a weekday amid daily rain.

The protesters have drawn attention because of their demands for reforms to Thailand’s constitutional monarchy, which they claim does not properly operate in a democratic framework.

That demand has caused a huge controversy because the royal institution has long been considered sacrosanct and a pillar of Thai identity. It is also protected by a lese majeste law that mandates three to 15 years in prison for defaming the monarchy.

Conservative royalist Thais accuse them of seeking to end the monarchy, an allegation they deny.

Efforts by several royalist groups to counter the previous rallies fell flat, but the turnout of counter-demonstrators was much larger Wednesday.

The protest movement was launched in March by university students but quickly put on hold as Thailand was gripped by surges in coronavirus cases. It came back in July, when the threat from the virus eased, and since then has again been spearheaded by students and publicized on social media.

The movement’s original core demands were new elections, changes in the constitution to make it more democratic, and an end to intimidation of activists.

The protesters charge that Prime Minister Prayuth, who as army commander led a 2014 coup that toppled an elected government, was returned to power unfairly in last year’s general election because laws had been changed to favor a pro-military party. Protesters say a constitution promulgated under military rule is undemocratic.