Nine pro-democracy campaigners are on trial in Hong Kong over their role in the “umbrella movement”, four years after the unprecedented protests gripped the city. They are pleading not guilty and hope to use the hearings to turn a spotlight on the issues behind their case: what is also on trial, says the defendant Benny Tai, a legal scholar, is the high degree of autonomy and the rule of law that Hong Kong is supposed to enjoy.

Unusually, prosecutors have based the charges on a common law offence which renders them more ambiguous than similar charges under statutory law and ensures that they carry far harsher sentences, of up to seven years. The defendants are accused of inciting public nuisance; Mr Tai and the two fellow founders of the Occupy Central campaign are also accused of conspiracy to cause public nuisance and, absurdly, inciting others to incite public nuisance. Amnesty International describes the charges as deliberately vague and designed to chill.

The trio’s initial call for a campaign of peaceful civil disobedience was soon overtaken by a student-led movement which was almost comically polite and well-behaved. The 79-day protests galvanised the city and crystallised many people’s frustrations with the growing encroachments upon its freedoms and way of life, guaranteed until 2047 under the joint declaration on the former British colony’s return to China. But it failed in its immediate political goal, of making the promise of universal suffrage in elections for the chief executive meaningful, and accelerated the clampdown. The last four years have seen the kidnapping and detention of Hong Kong booksellers, and the disqualification of pro-democracy legislators for amending their oaths of office, resulting in the pro-democracy camp losing its veto power.

Last year, student leaders were jailed for their role in the protests – after prosecutors appealed against what they called the “rather dangerous” leniency of their original sentences – thereby barring them from elections for five years. The best-known of them, Joshua Wong, had already warned that the promise of “one country, two systems” had become in reality “one country, one-and-a-half systems”.

Since then, Hong Kong has banned a pro-independence party as a “real threat to national security” and introduced a law criminalising disrespect of the national anthem. There has been growing pressure on and interference in academia, the judiciary and the media: last month, the government refused to review the work visa of a resident Financial Times journalist after he chaired a talk by a pro-independence advocate. Independent bookshops have closed.

As MPs have pointed out – to the displeasure of Hong Kong’s leadership, which complains of “highly undesirable” interference – the deterioration in the city’s autonomy is an issue for the British government, as the other party to the joint declaration. Despite Beijing’s open disdain for this document, this is a legally binding treaty lodged with the UN. The Foreign Office says individual trials are a matter for Hong Kong, but hopes the proceedings will not discourage lawful protest. The growing pressure on Hong Kong’s freedoms is a test for Britain too.