Songphon 'Yajai' Sonthirak had been a boxer for most of his young life and knew how to face down a stronger opponent: Keep your guard up, stand tall, stay focused. So he was shocked to discover, on the day of his arrest, how little his training mattered and how fear seized him.

Yajai, meaning “balm for the heart,” is a 21-year-old law student with a mop of dyed blue-green hair. He’d volunteered to help with security at a small anti-government protest in the heart of old Bangkok.

It was Oct 13, a Tuesday, the start of 11 days that would change Yajai’s life and shake Thailand.

For three months, thousands of young Thais had been pouring onto streets in protests of growing size and boldness, largely unimpeded by police. They were demanding the resignation of Prime Minister Prayuth Chan-ocha, a retired army chief who seized power in a 2014 coup.

Far more remarkably, they were publicly criticising King Maha Vajiralongkorn, who is protected from insult by a strict lese majeste law. The young demonstrators were thus risking criminal prosecution and tearing up the rules of a society centred on devotion to its monarch, who is still revered by many as semi-divine.

Yajai and his friends were gathered around a pick-up truck that doubled as a sound stage...