The race for chief executive was thrown open when unpopular hardline leader Leung Chun-ying declared last week he would not seek reelection after a term marked by political crises and anti-Beijing protests.
Since then, finance secretary John Tsang, considered more moderate, has resigned from the government and is widely expected to declare his leadership bid.
But he has been beaten to it by Regina Ip, who today announced her "Win Back Hong Kong" campaign.
She said she still wants to implement the controversial legislation at the same time as striving for social harmony in Hong Kong.
The deputy of Ip's New People's Party, Louis Shih, said Ip would be the "iron lady" that Hong Kong needs as she announced her candidacy at the city's harbourfront convention centre.
Ip said she was willing to work with non-establishment parties "as long as they faithfully uphold national sovereignty, territorial integrity and the Basic Law (Hong Kong's constitution)".
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