China has demanded Taiwan renounce any desire for independence after voters elected a president who considers the island a sovereign country and whose ascension creates new uncertainty in a region long riven with tensions and threats of war.

Former law professor Tsai Ing-wen won a landslide victory in nationwide ballots Saturday, becoming the first woman elected president of a Chinese nation. She seized the second-largest margin of victory in a Taiwanese presidential election and also led her Democratic Progressive Party (DPP) to its first majority in the legislature, where it took 68 out of 113 seats.

The vote was a stunning loss for the Kuomintang (KMT), the party that has ruled Taiwan for most of the past seven decades, but also served as a popular repudiation of outgoing president Ma Ying-jeou, who made warmer relations with China a central objective of his time in office. The KMT was left with just 35 seats.

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Ms. Tsai's win thrusts Taiwan into a less predictable new era of relations with Beijing, which considers the island a renegade province and is prepared to keep it as part of "one China" by using force.

Her "rise to power will likely set back hard-earned cross-Straits relations, which could worsen the island's flagging export-dependent economy and narrow its diplomatic space in the international community," warned Wang Jianmin, an expert in cross-straits affairs at the Chinese Academy of Social Sciences, the country's premier research institution, in comments to the Communist Party-run Global Times.

Shortly after the votes were counted, China's Taiwan Affairs Office said Beijing will "resolutely oppose any form of secessionist activities seeking 'Taiwan independence.'"

The office said that "on major matters of principle including safeguarding national sovereignty and territorial integrity, our will is rock-firm and our attitude is consistent."

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But the comments largely restated Beijing's long-held position, and suggested an attempt to greet Ms. Tsai with a degree of accommodation not accorded the first DPP president, Chen Shui-bian, who angered both China and the United States by seeking a new Taiwan constitution.

Three days before Mr. Chen was sworn into office for his second term in 2004, Beijing warned that "if Taiwan leaders should move recklessly to provoke major incidents of 'Taiwan independence,' the Chinese people will crush their schemes firmly and thoroughly at any cost."

Mr. Chen, however, never controlled the legislature, and won with much smaller margins than Ms. Tsai. Her decisive victory poses a new challenge for China, which for the first time faces strong political control in Taiwan by a party that has leaned toward independence.

The president-elect, her voice hoarse from a long campaign, struck a confident note in her first postelection remarks. She warned Beijing against harassing Taiwan amid anger over the treatment of a young pop star who was forced by mainland outrage to make a quivering apology for holding the Republic of China flag used in Taiwan.

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"Our democratic system, national identity and international space must be respected. Any forms of suppression will harm the stability of cross-strait relations," Ms. Tsai said.

She pledged to "maintain the status quo for peace and stability across the Taiwan Strait," but also to protect "this country's sovereignty."

Ms. Tsai's election platform was founded on domestic concerns such as a stagnant wages and food safety, and she owed her win in part to popular dissatisfaction with Mr. Ma, the outgoing president plagued by single-digit approval ratings.

But for many voters, the ballot decision was "largely about identity," said Huang Chang-ling, a political scientist at National Taiwan University. Polls show a growing percentage of people identify as Taiwanese rather than Chinese.

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Hu Ching-chien, a 32-year-old engineer, was among those who came out under sunny skies in Taipei Saturday to vote for Ms. Tsai. "I feel I have finally accomplished something for Taiwan, so that Taiwan will have greater autonomy," he said. "If there was no change, Taiwan would become just another part of China."

As votes were counted Saturday night, a large cheering crowd gathered outside the DPP election headquarters where someone had erected a large sign saying: "Taiwan is NOT Part of China! Support Taiwan Independence." One supporter held a flag that declared "I am Taiwanese. I stand for Taiwan's Independence."

That sentiment has also gained new footing inside Taiwan's legislature, with the election for the first time of the New Power Party, a youthful new political force that gained five seats and whose members talk openly about independence.

With Ms. Tsai's election, "we are entering new territory," said Richard Bush, the former head of the de facto U.S. embassy in Taiwan who is now director of the Brooking Institution's Center for East Asia Policy Studies.

The trend on the island appears to be toward "a fear of mainland intentions and anxiety about being too dependent on China and fear of being on a slippery slope toward unification and domination," he said.

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And, he added, Beijing's distrust of Ms. Tsai raises the spectre "of a downward spiral" in cross-strait relations.