BANGKOK (BLOOMBERG) - Thai protest leaders are planning the biggest gathering since the 2014 military coup, as they seek to build momentum in months-long demonstrations for greater democracy and reform of the country's powerful monarchy.

As many as 100,000 protesters are expected at Thammasat University campus in Bangkok on Saturday (Sept 19), according to the student-led group organising the event, the United Front of Thammasat and Demonstration.

Although the university said last week it wouldn't allow protesters on campus, the group vowed the gathering would proceed as planned.

Over the past months, the protest movement led by mostly students, has expanded, with various groups using digital spaces to set meetings and spread their demands in a style of organising reminiscent of the leaderless flash mobs in Hong Kong and the US.

The demonstrators are calling for changes in the Constitution that would remove the military influence in politics and for a new, more democratic election.

"The youth movement has the potential to gain momentum and include the middle class, working class and people outside of Bangkok as part of the protests," said Assistant Professor Prajak Kongkirati, the head of the politics department at Thammasat University. "It's adding more pressure to the government that's losing its popularity and struggling to boost economic confidence."

The mounting protests present a challenge for Prime Minister Prayuth Chan-o-cha, a former army chief who led a 2014 coup and stayed in charge after a disputed election last year conducted under rules written by his military government. The premier has said the government was looking into possible amendment of the charter that was drafted by the junta ahead of last year's vote.

Mr Prayuth's government is currently without a Finance Minister after the previous chief resigned less than a month into the job, and it's facing a two-week delay in its fiscal year 2021 budget amid slow stimulus spending. The Thai economy, heavily reliant on trade and tourism, is on course for its deepest ever annual contraction of 8.5 per cent this year.

On Thursday, the prime minister appealed to the protesters to consider the risk of virus infection the gathering could trigger.

"Any major flare up of infections will lead to terrible consequences and even worse economic destruction, the likes of which we have never seen," he said in televised comments.

Mr Prayuth has put Deputy Premier Prawit Wongsuwan in charge of the security for the weekend protest, who said the security forces will avoid confrontation and violence and he's confident the situation will be under control.

'ROOT CAUSE'

Last month, the Thammasat student-led group made 10 demands, including a call for revoking the country's strict lese-majeste laws that criminalise insults against top members of the royal family. The demonstrators are breaking deeply entrenched taboos in Thailand, where openly criticising the monarchy can lead to long jail sentences.

Other demands include changing the Constitution to allow criticism of the king, separating the monarch's properties from the Crown Property Bureau, aligning the budget for the monarchy with economic conditions, banning the monarch from expressing political opinions and prohibiting the king from endorsing coups.

"We have to talk about these issues because they're the root cause of problems in Thai politics," Mr Parit Chiwarak, one of the student activists of the Thammasat group, said in a phone interview. He wants protesters to camp overnight to "send a signal that the people want to take back control."

Since the protest movement began in July, 14 people have been arrested by police and later released on bail, according to data from the Thai Lawyers for Human Rights. At least 61 people faced charges for leading or participating in the protests, with some of them facing sedition charges which can lead to up to seven years in jail.