A prominent businesswoman has sparked outrage in Hong Kong by comparing the city's pro-democracy protesters to African-American slaves, suggesting they might need to "wait for a while" to win the freedoms they are seeking.

"American slaves were liberated in 1861 but did not get voting rights until 107 years later, so why can't Hong Kong wait for a while?" Laura Cha, an HSBC board member, was quoted as saying by The Standard newspaper on Friday.

An online petition calling for her to apologise, and for her fellow HSBC board members to denounce the comments, has gathered nearly 7,000 signatures.

Pro-democracy demonstrators have held mass street rallies in Hong Kong for more than a month, demanding Beijing grants free leadership elections to the semi-autonomous Chinese city.

The Chinese government insists that candidates for the 2017 vote must be vetted by a pro-Beijing committee, which the protesters say amounts to "fake democracy".

Cha -- who also sits on Hong Kong's Executive Council, which advises leader Leung Chun-ying -- has said she did not mean any disrespect with the comments and "regrets" that they have caused concerns.

But the online petition described the remarks as "deeply insulting", as well as showing a "lack of understanding of American history".

"We find it extremely distasteful and insensitive to compare the voting rights of average, natural-born citizens of Hong Kong to the path of voting rights of slavery," wrote Jeffrey Chan, who started the petition on Change.org.

The protesters have occupied several major thoroughfares in the former British colony since September 28.

Photographs circulating on social media Saturday showed the city's last colonial leader Chris Patten holding an open yellow umbrella -- the symbol of the pro-democracy movement -- in an apparent gesture of support at Oxford University, where he is chancellor.