A police officer detains a demonstrator during the protest against proposed extradition bill with China in Hong Kong on Sunday. Thomas Peter / Reuters

Hong Kong was plunged into a fresh political crisis on Sunday night after hundreds of thousands of people took to the streets to thwart a proposed extradition law that would allow suspects to be sent to mainland China to face trial.

After seven hours of marching, organisers estimated 1,030,000 people took part, far outstripping a demonstration in 2003 when half that number hit the streets to successfully challenge government plans for tighter national security laws.

A police spokesman said police estimated 240,000 were on the march "at its peak."

Sunday's outpouring was already raising the pressure on the administration of Hong Kong Chief Executive Carrie Lam and her official backers in Beijing.

"She has to withdraw the bill and resign," veteran Democratic Party lawmaker James To told crowds gathering outside the city's parliament and government headquarters in the Admiralty business district on Sunday night.

"The whole of Hong Kong is against her."

After To spoke, thousands were still arriving, having started the march five hours earlier, filling four lanes of a major thoroughfare.

Some sat in a nearby park singing "hallelujah", but on a nearby road tensions were building after hours of peaceful protest.

Riot police armed with batons and helmets were gathering while government-funded broadcaster RTHK reported that they used pepper spray on six masked men trying to block the road.

Lam had yet to comment on the rally and the future of the bill, which is due to be debated in the Legislative Council on Wednesday and could be passed into law by the end of June.

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

The demonstration capped weeks of growing outrage in the business, diplomatic and legal communities, which fear corrosion of Hong Kong's legal autonomy and the difficulty of ensuring basic judicial protections in mainland China.

US and European officials have issued formal warnings - concern matched by international business and human rights lobbies that fear the changes would dent Hong Kong's rule of law.

The former British colony was handed back to Chinese rule in 1997 amid guarantees of autonomy and various freedoms including a separate legal system, which many diplomats and business leaders believe is the city's strongest remaining asset.

The unusually broad opposition to the rendition bill displayed on Sunday came amid a series of government moves to deepen links between southern mainland China and Hong Kong.

Chants of "no China extradition, no evil law" echoed through the highrise city streets as marchers snaked through the Causeway Bay and Wanchai shopping districts, while marchers called for Lam and other senior officials to step down.

Some carried yellow umbrellas - a symbol of the pro-democracy Occupy protests that choked key city streets for 79 days in 2014.

Reuters