The U.S. expressed "grave concern" over Hong Kong legislation that would for the first time allow extraditions to mainland China, raising pressure on Beijing as the city braced for a potentially historic showdown over the proposal.

The bill "could undermine Hong Kong's autonomy and negatively impact the territory's long-standing protections of human rights, fundamental freedoms and democratic values," State Department spokeswoman Morgan Ortagus told reporters Monday in Washington. Opponents of the legislation Sunday staged one of the largest protests since the former British colony's return to China: Organizers said more than 1 million participants showed up, while police put the figure at 240,000.

Tensions are only heating up, with activists calling for strikes and vowing to surround the city's Legislative Council on Wednesday, when lawmakers debate scores of proposed amendments.