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

Organisers said their initial estimates put the turnout at well over half a million people, saying it outstripped a demonstration in 2003 when 500,000 hit the streets to challenge government plans for tighter national security laws.

Those laws were later shelved and a key government official forced to resign.

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 outsidethe city’s parliament and government headquarters 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 amajor thoroughfare.

Some sat in a nearby park singing”hallelujah”, but on a nearby road tensions were building afterhours 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.

The demonstrationcapped weeks of growing outrage in the business, diplomatic andlegal communities, which fear corrosion of Hong Kong’s legalautonomy and the difficulty of ensuring basic judicialprotections in mainland China.U.S. and European officials have issued formal warnings -concern matched by international business and human rightslobbies 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 businessleaders believe is the city’s strongest remaining asset.

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

Police had yet to issue their own estimate of the protest size.

But as tens of thousands reached the Legislative Councilin the Admiralty business district, the starting point inVictoria Park was crowded with thousands more still waiting to join the march.

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

Streets were packed along the route; Reuters witnesses atvarious key points estimated the crowd at several hundred thousand.

Chants of “no China extradition, no evil law” echoed throughthe highrise city streets, while other marchers called for Lamand other senior officials to step down.