Demonstrators have taken to the streets of Hong Kong to protest a proposed law that would allow suspects to be transferred and tried in courts of law on mainland China.

Organizers said on Sunday they expected to draw more than 500,000 people to the demonstration, making it the biggest protest in years.

Security officials called for public restraint as more than 2,000 police officers were deployed to Hong Kong to maintain order.

Municipal officials shut down the busy downtown Wanchai and Mongkok metro stations in an effort to dissuade masses from heading to Victoria Park, the starting point of the march.

Carrie Lam, who took office as chief executive of Hong Kong in 2017, has defended the bill. She said the legislation was necessary to plug a longstanding loophole.

Sunday’s march comes amid renewed calls for deeper electoral reforms stalled five years ago after the Occupy Movement prompted officials to shelf proposed reforms to the Hong Kong electoral system.

Thousands protest proposed extradition law in Hong Kong

Thousands of people march on Hong Kong’s parliament to demand the scrapping of proposed extradition rules that would allow people to be sent to mainland China for trial.

Under a deal between Britain and China in 1997, Hong Kong was reunited with China, but it was decided that the territory should continue to enjoy its freedoms, including a separate legal system.

China says those privileges have been protected but insists it could not tolerate movements that seek full secession from the mainland.

Washington has voiced concern over Hong Kong’s extradition treaty with China, prompting Beijing to denounce gross interference by the United States.