Imagine that 5 to 10 million Britons took to the streets to attack new legislation. Imagine that the previously cautious, passive or politically conservative came out, including lawyers and church congregations. Then imagine that – even with further protests pending – the government said they just didn’t understand the issue, and vowed to ram through changes within days.

This is what Hong Kong has just seen. As many as a million residents, equivalent to one in seven of the population, marched against extradition changes. The turnout, astounding even to organisers, reflected the understanding that these amendments are the latest and most decisive breach of China’s commitment to maintain Hong Kong’s way of life for 50 years after its handover from Britain in 1997. By removing the firewall between the legal systems, the changes expose both Hong Kong residents and those who travel there to the whims of the Chinese Communist party, which controls the mainland’s courts and is handy at mustering convenient legal cases.

The pretext for the changes is the need to address a murder case in Taiwan, which would not in fact be resolved by them. Whatever the Hong Kong chief executive, Carrie Lam, says, the assumption is that the extradition changes, which would allow the accused to be transferred to the mainland without legislative oversight, are designed to suit Beijing. Opponents predict the law will be used against critics, and potentially much more widely – perhaps against people caught in commercial disputes – and point to the cases of a bookseller and a tycoon who had foreign passports but were kidnapped from Hong Kong.

On Wednesday, the protests will continue, with walkouts by school and university students, bus drivers going slow and a sit-in near government buildings. Hundreds of businesses, including bakeries and toy shops, have said they will shut.

Most are realistic about their prospects of success. Last Sunday’s demonstrations were probably the biggest since the handover, and certainly since 2003, when rallies led to the shelving of draconian national security laws. But the protests are a sign of desperation more than hope: a visceral expression of fear, anger and pride in Hong Kong’s identity. Denied a meaningful electoral say by their very limited form of democracy, many want to vote with their feet. According to one survey, one in three Hong Kong residents, and one in two young people, want to emigrate, with political issues among the top push factors.

Since Ms Lam insists this is a local initiative, Beijing could, in theory, shelve the proposals and let her deal with the consequences. In reality, this would allow Beijing to carry on abducting those it really wants, probably to minimal protest. Foreign companies would be reassured: Hong Kong’s economic importance to China is much diminished, but with international businesses unsettled by the trade war, it may be more valuable than in recent years.

Under Xi Jinping the mainland has seen ever-increasing repression, and since 2014’s peaceful umbrella protests the Hong Kong authorities have made clear their determination to crush any sign of opposition. They have disqualified a growing number of candidates, banned a political party peacefully promoting independence, and ousted elected representatives. They have prosecuted dozens of pro-democracy leaders. Absurdly, a law professor who co-founded the 2014 movement has been sent to a maximum security prison. Independent bookstores have closed and others have stopped selling works critical of China. A draft law criminalises disrespecting the Chinese national anthem.

Meanwhile, Beijing has shown its contempt for opinion in Hong Kong and abroad. It has described the legally binding Sino-British joint declaration on the return of Hong Kong as no longer of “any practical significance”. Some might conclude further representations are pointless. But Hong Kong’s demonstrators have made their stand regardless of the outcome, and they deserve support. Beijing’s attitude is all the more reason for leaders overseas, and especially in Britain, to urge it to respect Hong Kong’s autonomy in clear and forceful terms.