Tsai called for Taiwan to "consolidate its sovereignty" as she formally registered to become the Democratic Progressive Party's (DPP) candidate for next year's vote, for which she is likely to stand uncontested.
The 58-year-old party chairwoman failed to unseat incumbent President Ma Ying-jeou of the Beijing-friendly Kuomintang party (KMT) when she ran against him for the top post in 2012.
But recent polls have given Tsai a strong lead for the 2016 contest with the KMT, which has seen its popularity slump because of growing public concern over its warming ties with China.
"As the leader of the country, the most critical task is to consolidate its sovereignty," Tsai told reporters today.
"Lots of people might have been disappointed by Taiwan's political culture and distrust the government. People need a new option."
A legal scholar-turned-politician who served as vice premier in the former DPP government, Tsai turned the party around after it suffered a crushing defeat in 2008 dogged by a string of corruption scandals.
A proposed trade pact with the mainland sparked mass student-led protests and a three-week occupation of Taiwan's parliament last year.
A United Daily News poll last week found 65 percent of respondents expected the DPP to win the presidential vote and only a tenth think the KMT has a chance of retaining power.
"Taiwan needs reforms but there will be pain accompanying the reforms," Tsai wrote on her Facebook page yesterday.
"We should create an era in politics that truly belongs to the people, a new politics of transparency, cleanness, participation, tolerance.