She is the very embodiment of modern Thai youth, dressed in flip-flops, T-shirt and shorts, and sipping an iced coffee with friends after university lectures. But 20-year-old Kanthoop is not just another university student. The social welfare major has been spat at, publicly denigrated, threatened by police and faces up to 15 years in jail – for little more, she says, than "having opinions".

"I know my case is symbolic, and I'm happy about that. There is good that comes from somebody standing up and wanting to make change – sooner or later people will start to realise that."

To understand why Kanthoop might be so vilified is to understand Thai society. Twice a day – at 8am and 6pm – time stands still in this nation of 69 million as the national anthem sputters out of public loudspeakers and everyone is expected to stand in silent salute.

The routine testifies to the adoration Thai people feel for both their nation and their king, Bhumibol Adulyadej, or Rama IX, a man so revered that many shops and homes bear his portrait. But that reverence is backed up by the world's strictest pro-monarchy regulations, which sentence anyone who insults, defames or threatens the king or his family to three to 15 years' imprisonment.

Opponents argue that the law, known as Article 112, prevents healthy dialogue and is being used as a political tool to stifle dissent. Charges of lese-majesty, though in existence since 1908, have jumped since the military coup in 2006 that ousted former PM Thaksin Shinawatra, who was widely criticised for attempting to undermine the monarchy, an accusation he has long denied.

In 2010 – when royalist forces bloodily battled with Thaksin supporters – 478 lese-majesty charges were made and 75,000 websites blocked. Human rights groups, as well as the US, EU and UN, have voiced concern over the way the law is used.

A group of Thai academics and activists, called Nitirat, have since proposed amendments to the law, but current prime minister, Yingluck Shinawatra, Thaksin's sister, has vowed not to touch 112 — promising in January to "protect the [royal] institution, not exploit it". The debate has consequently been left to rage in the streets, where Nitirat's members face threats and harassment by royalists.

"This is about national security, not just about the king," said royalist Dr Tul Sittisomwong. "Thai people are not that well educated … We're not that open to layers of discussion without fear of violence [regarding this subject]. The king makes peace in our society."

But the existing "hyper-royalism" in Thailand has spiralled out of control and may actually be working to the detriment of the nation, said Thongchai Winichakul, a professor of south-east Asian history at the University of Wisconsin-Madison who spent two years in prison after participating in a 1976 pro-democracy protest that saw over 100 demonstrators killed.

"Just look at the hyperbole [used] to describe the monarchy, the religiosity with which Thai people love the monarch and the public participation of all this royalism," he says. "People are now afraid of their colleagues" — because anyone can bring forward a charge of lese-majeste, he adds.

It's an issue that Kanthoop knows well. Police began investigating her case in 2010 after she posted Facebook messages that were later cut and pasted by others, who she says distorted what she wrote and forwarded it by email to authorities. At her police summons on 11 February, Kanthoop was told that her case had been postponed to an unspecified date while police gather more evidence.

If charged, she may well be 112's youngest offender, but she will probably not be the last. Last week a Thai court sentenced a 71-year-old redshirt supporter to 7½ years in prison, while last year a 61-year-old was jailed for 20 years for sending defamatory text messages, and a Thai-US citizen was jailed for 2½ years for translating a banned biography of the king.

Kanthoop's political journey began in 2006, when she refused to stand up in the cinema for the royal anthem that plays before every film. "That was the moment for me," she says. "I decided that I have the right to stand up or not, to pay respect to whatever I believe in."

While her highly politicised views have not won her many friends at Bangkok's Thammasat University, the only university to accept her even though she passed the entrance exams for two others, Kanthoop is not alone in her fight. A group of activists recently went on hunger strike outside the capital's criminal court to demand that those detained on charges of lese-majesty be granted bail. "This law needs to be reviewed," says 20-year-old Panitan Pruksakasemsuk, whose father Somyot is one of those detained. "Society needs to be open to change and willing to adapt to that change."

As for Kanthoop, while the future is uncertain, her approach to it is not. "If I have to go to jail, I will," she says calmly. "Even if it's for life. But I won't plead guilty to reduce my sentence, and I won't ask for the king's pardon. I am guilty only of freedom of thought."

• This article was amended on 7 March 2012. The original referred to the postponement of a 'court date' for Kanthoop. This has been corrected.