It’s hard for “K” to pinpoint the exact moment he believes he suffered a brain haemorrhage during a violent arrest by two undercover officers in Hong Kong’s main shopping district at the end of a long day of pro-democracy protests in August.

The Hong Kong teenager, too afraid to be named, has now returned to his studies in the UK, but the memories of that terrifying, humid night – a blur of batons repeatedly striking his head, back and arms as his body was slammed to the ground, hands tied with plastic cuffs, still haunt him.

Recently turned 19, he claims the officers did not identify themselves or offer a reason for his arrest as they carted him off to the notorious San Uk Ling detention centre and denied him timely legal and medical assistance. Regular nosebleeds are now a reminder of his physical and mental trauma.

During nearly five months of civil unrest that have rocked the city of 7.5 million, K’s experience is not isolated.

Protesters, lawyers, educators, politicians have made multiple allegations against the police of excessive force, abuse of power and a collapse of accountability as unidentifiable officers, their badges hidden, appear to breach the city’s law and their own rules of conduct.