Scores of Hong Kong businesses have vowed to shut down as anger builds over government plans to push through a law allowing extraditions to China, despite a huge weekend protest.

The financial hub was rocked by a huge rally on Sunday -- the largest since the city's 1997 return to China -- as vast crowds called on the city's leaders to scrap the Beijing-backed plan.

Many are fearful that the proposal will tangle people up in the mainland's opaque courts and hammer Hong Kong's reputation as an international business hub.

Organisers said more than a million hit the streets but the record crowds have failed to sway chief executive Carrie Lam who has rejected calls to withdraw or delay the bill.

On Wednesday the proposed law will have its second and third readings in the city's parliament, which is dominated by Beijing loyalists, making its passing all but assured.

Protest groups have vowed to stage a fresh rally outside parliament that day and have urged people to join or to go on strike.

Business owners have taken to social media using a hashtag that translates as "#612strike" to announce solidarity closures, allowing staff to join the protest.

A large chunk are mom-and-pop style stores and small businesses that are a key part of the city's economy, but which often eschew the city's raucous street politics.

By Tuesday morning, more than 100 businesses had declared plans to strike, ranging from coffee shops and restaurants to camera stores, toy shops, nail salons, yoga studios and even an adult entertainment store.

"Hong Kong was built by our various generations with hard work," wrote Meet Yoga studio on its Instagram account. "A Hong Kong without freedom -- how about we just wipe it off the map entirely and call it China?"

One owner of a floral accessories store added: "I’m a Hong Kong girl who doesn’t know politics and finds pleasure in the small things of life. But even I know politics actually affects all aspects of our lives."

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Lawyer Michael Vidler said he would allow his 12 employees to "act in accordance with their conscience" and go on strike.

The proposed law would allow extraditions to any jurisdiction with which it does not already have a treaty -- including mainland China.

Hong Kong's leaders say it is needed to plug loopholes and to stop the city being a sanctuary for fugitives.

They say safeguards are in place to ensure human rights standards are upheld and that political critics of Beijing will not be targeted.

But many Hong Kongers have little faith in those assurances after years of political unrest caused by heightened fears a resurgent Beijing is trying to quash the city's unique freedoms and culture.