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.