Hundreds of thousands protested in Hong Kong on Sunday against a government plan that would allow extraditions to mainland China. The mass demonstration was among the largest in Hong Kong’s history and another sign of rising fear and anger over the erosion of the civil liberties that have long set the semiautonomous city apart from the Chinese mainland. On Wednesday, the government delayed debate of the Bill following further protests involving tens of thousands of people.

The relationship between Hong Kong and the central government in Beijing is complicated and evolving. Here’s the key background.

Is Hong Kong part of China?

Yes, but it’s not that simple. Hong Kong, a former British colony, was returned to China in 1997 under a policy known as “one country, two systems”, which promised the territory a high degree of autonomy. The policy has helped preserve Hong Kong’s civil service, independent courts, freewheeling press, open internet and other features that distinguish it from the Chinese mainland.

But that autonomy, guaranteed under a mini-constitution known as the Basic Law, expires in 2047. Well before Hong Kong is set to lose its unique status, however, the Basic Law has been weakened as China’s Communist Party increasingly meddles in Hong Kong’s affairs – for example, by abducting booksellers and a Chinese-born billionaire.

Why is Beijing meddling in Hong Kong?

The pressure reflects a broader tightening of controls across China under President Xi Jinping, who came to power in 2012 and has pursued critics with increasing boldness. Hong Kong is an obvious target because it has a vocal community of pro-democracy activists and lawmakers. Tens of thousands took part in a movement demanding free elections that seized control of downtown streets for 11 weeks in late 2014, and large crowds attend an annual vigil that commemorates Beijing’s 1989 crackdown on Tiananmen Square.

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But the Basic Law guarantees that Chinese authorities cannot stifle dissent in Hong Kong with an iron fist, as they do across the mainland and in the autonomous regions of Tibet and Xinjiang. Analysts say that has forced Beijing to chip away at the independence of Hong Kong’s institutions by other means – for example, by pressing the extradition plan.

Protesters shelter under umbrellas during a downpour as they occupy roads near the government headquarters in Hong Kong. Photograph: Anthony Wallace/AFP/Getty Images

What is the extradition plan?

The Bill would allow Hong Kong to detain and transfer people wanted in countries and territories with which it has no formal extradition agreements, including Taiwan and the Chinese mainland. Carrie Lam, Hong Kong’s chief executive, has said the new law is urgently needed to prosecute a Hong Kong man who is wanted in Taiwan for the murder of his girlfriend.

Critics contend that the law would allow virtually anyone in the city to be picked up and detained in mainland China, a country in which judges must follow the orders of the Communist Party. They fear the new law would not just target criminals but political activists as well.

The extradition plan applies to 37 crimes. That excludes political ones, but critics fear the legislation would essentially legalise the sort of abductions to the mainland that have taken place in Hong Kong in recent years. (The mainland authorities are typically not permitted to operate here.)

Under the law, the chief executive would need to approve an extradition request before an arrest warrant is issued. A Hong Kong court would also be empowered to check that there is a basic case against a suspect. Yet Hong Kong’s subordinate status to the mainland would make it extremely difficult for a local leader to reject an extradition request from her superiors. Taiwan, a self-governing democracy, has said it would not comply with any extradition agreement that lumps it together with the Chinese mainland. And many in Hong Kong – where the government has ousted opposition lawmakers and rejected demands for free elections – see the extradition plan as the endgame of a long battle to disable dissent and political opposition in their city.

Who opposes the plan?

The plan has sparked petitions from people across Hong Kong who fear they could end up in a mainland legal system where the Communist Party routinely prosecutes dissidents and others for political reasons. Hong Kong is connected to the Chinese mainland by a land border, high-speed rail and a long sea bridge. Residents on both sides cross the border regularly, although mainlanders must apply for permission to enter Hong Kong.

An April demonstration against the plan was the city’s biggest in five years, and anger over it led to scuffles in Hong Kong’s legislature last month. Sunday’s protest, which stretched for more than a mile through canyons of downtown skyscrapers, may have been the largest since 2003, when half a million marchers demonstrated against a Beijing-backed package of national security laws prohibiting sedition, subversion and treason against the Chinese government.– New York Times