Reuters reports the appeal for US intervention was made during a march in front of the US Consulate: 

Thousands of protesters earlier sang the Star Spangled Banner and called on U.S. President Donald Trump to "liberate" the city. They waved the Stars and Stripes and placards demanding democracy.

The demonstrators directly handed petitions to US consular officials reportedly while chanting "Fight for freedom, stand with Hong Kong," and further others which said "Resist Beijing, liberate Hong Kong."The call for US "intervention" comes interestingly even after HK leader Carrie Lam publicly announced Wednesday she was formally withdrawing the controversial extradition bill, which sparked the protests earlier this summer in the first place. Sunday's protesters called for the passing of the proposed "Hong Kong Human Rights and Democracy Act 2019" by US Congress and handed out flyers and petitions.