Taiwanese President Tsai Ing-wen resigned as leader of the ruling Democratic Progressive Party after its poor performance in local elections.
The opponent of the Kuomintang (KMT) won several major races on Saturday, notably in the capital Taipei.
The vote has drawn worldwide attention as Taiwan becomes a bigger geopolitical hot spot between China and the United States.
President Tsai had framed the election as a vote for democracy amid growing tensions with China.
“The election results were not as expected… I must take full responsibility and am resigning immediately as chairperson of the DPP,” Ms Tsai, who will remain president of the self-governing island, told reporters.
Elections for local councils and city mayors are nominally national in focus, covering issues such as crime, housing and welfare, and elected officials will have no say in Taiwan’s politics in the future. regard to China.
However, Ms Tsai and government officials have urged voters to use the election to send a message about defending democracy as Beijing increases pressure on the island.
Voters also rejected lowering the voting age from 20 to 18 in a referendum held alongside local elections.
The Chinese government sees Taiwan as a breakaway province that will eventually become part of the country.
But many Taiwanese views their self-governing island – with its form of government and democratic system – as distinct.
Tensions peaked in August when Beijing held huge military drills around Taiwan to protest the visit of US House Speaker Nancy Pelosi to the island.
The United States has long walked a tightrope over Taiwan. Officially, he has no formal ties to Taiwan but also pledged to supply the island with defensive weapons and stressed that any attack from China would cause “serious concern”.