Several hundred thousand people jammed Hong Kong's streets today in a last bid to thwart a proposed extradition law that would allow suspects to be sent to China to face trial, with police bracing for the biggest march in the city in 15 years.

Police chiefs called for public restraint, government-funded broadcaster RTHK reported, as they mobilised more than 2,000 officers for a march that organisers expected to draw more than half a million people.

That would make it the biggest rally since a similar number turned out in 2003 to challenge government plans for tighter national security laws, which were later shelved.

Police officers detain a demonstrator during a protest to demand authorities scrap a proposed extradition bill with China in Hong Kong in China on June 9, 2019. Photo: Reuters

Police and organisers had yet to give estimates, but as tens of thousands reached the Legislative Council in Admiralty business district, the starting point in Victoria Park was crowded with thousands more still waiting to join the march.

Streets were packed along the route, with Reuters witnesses at various key points estimating the crowd at several hundred strong.

Debates start at the council on Wednesday over the amendments to the Fugitive Offenders Ordinance. The bill could be passed into law by the end of June.

Chants of "no China extradition, no evil law" echoed through the high-rise city streets, while other marchers called for Hong Kong's Chief Executive Carrie Lam to step down. One protester held a sign reading "Carry off Carrie".

Lam has tweaked the proposals but has refused to withdraw the bill, saying it is vital to plug a long-standing "loophole".

The genial crowd included young families pushing babies in prams as well as the elderly braving 32 degree C (90°F) heat, some spraying each other with water misters.

Opposition to the proposed bill has united a broad range of the community, from usually pro-establishment business people and lawyers to students, pro-democracy figures and religious groups.

Insurance agents, executives and small entrepreneurs joined bus drivers and mechanics, with Reuters speaking to dozens of people saying it was their first protest march.

"I come here to fight," said a wheelchair-bound, 78-year-old man surnamed Lai, who was among the first to arrive.

School teacher Garry Chiu joined the protest with his wife and 1-year-old daughter, saying "it is no longer about me".

"I need to save my daughter. If the law is implemented anyone can disappear from Hong Kong. No one will get justice in China. We know there is no human rights," he said.

"The extradition bill will directly threaten the core values of Hong Kong and rule of law," said 21-year-old Kelvin Tam, a student in London. "It will remove the firewall of Hong Kong judicial independence."

Today's protests were also being planned in 25 cities globally, including London, Sydney, New York and Chicago.

People attend a protest against proposed extradition bill of Chinese-ruled Hong Kong with China, in Berlin in Germany on June 9, 2019. Photo: Reuters

The changes will simplify case-by-case arrangements to allow extradition of wanted suspects to jurisdictions, including mainland China, Macau and Taiwan, beyond the 20 that Hong Kong already has extradition treaties with.

But it is the prospect of renditions to mainland China that has alarmed many in Hong Kong. The former British colony was handed back to Chinese rule in 1997 amid guarantees of autonomy and freedoms, including a separate legal system.

"It's a proposal, or a set of proposals, which strike a terrible blow ... against the rule of law, against Hong Kong's stability and security, against Hong Kong's position as a great international trading hub," the last British governor, Chris Patten, said on Thursday.

Opponents of the plan say they deeply question the fairness and transparency of the Chinese court system and worry about security forces contriving charges.

Foreign governments have also expressed concern, warning of the impact on Hong Kong's reputation as an international financial hub, and noting that foreigners wanted in China risk getting ensnared in Hong Kong.

The concerns were highlighted on Saturday with news that a local high court judge had been reprimanded by the chief justice after his signature appeared on a public petition against the bill.

Reuters reported earlier that several senior Hong Kong judges were concerned about the changes, noting a lack of trust in mainland courts as well as the limited nature of extradition hearings.

Human rights groups have repeatedly expressed concerns about the use of torture, arbitrary detentions, forced confessions and problems accessing lawyers in China.

Hong Kong officials have defended the plans, even as they raised the threshold of extraditable offences to crimes carrying penalties of seven years or more.

They say the laws carry adequate safeguards, including the protection of independent local judges who will hear cases before any approval by the Hong Kong chief executive. No-one will be extradited if they face political or religious persecution or torture, or the death penalty.

"We continue to listen to a wide cross-section of views and opinions and remain to open to suggestions on ways to improve the new regime," a government official said today.