TAIPEI (Reuters, AFP) - Taiwan is an internal matter for China, there is only one China in the world and the island's election neither changes this reality nor international acceptance of it, China's government said after the pro-independence opposition won a landslide.

Tsai Ing-wen and her Democratic Progressive Party (DPP) won a convincing victory in both presidential and parliamentary elections on Saturday.

President-elect Tsai pledged to maintain peace with giant neighbour China, which claims Taiwan as its territory and has never renounced the use of force to bring it under its control.

In a short statement released just before midnight on Saturday, China's Foreign Ministry said no matter what changes there may be on the island, China would never change its policy of opposing Taiwan's formal independence.

"The Taiwan issue is an internal matter for China," it said. "There is only one China in the world, the mainland and Taiwan both belong to one China and China's sovereignty and territorial integrity will not brook being broken up," the ministry added. "The results of the Taiwan region election does not change this basic fact and the consensus of the international community."

China hopes the world will continue to uphold a "one China"principle, oppose any form of Taiwan independence and takes"real steps" to support the peaceful development of relations across the Taiwan Strait, it added.

Shortly after her victory, China's Taiwan Affairs Office warned it would oppose any move towards independence and that Beijing was determined to defend the country's sovereignty and territorial integrity.

In a strongly-worded editorial, Chinese state news agency Xinhua said the DPP's return to power "poses grave challenges to cross-Strait relations" and had "aroused concerns" about Taipei's relationship with Beijing.

Although Taiwan is self-ruling after it split with China following a civil war in 1949, it has never declared independence and Beijing still sees it as part of its territory awaiting reunification.

In her first comments to media, Tsai warned that Chinese "suppression" would damage ties with the mainland.

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

Support for Tsai has surged as voters have become increasingly uneasy about a recent rapprochement with China under outgoing KMT president Ma Ying-jeou.

Tsai's victory came on the same day that outrage erupted over the treatment of 16-year-old Taiwanese K-pop star Chou Tzu-yu, who was forced to record a video apology after angering Chinese netizens by flying a Taiwanese flag in a recent online broadcast.

Tsai specifically referred to Chou in her address, saying her case had "shaken Taiwanese society".

"This particular incident will serve as a constant reminder to me about the importance of our country's strength and unity to those outside our borders," she said.

Tsai has toned down the DPP's traditionally pro-independence message to assuage Beijing and calm nerves in the United States - Taiwan's major ally - which does not want to see tensions flare.

In her address to media she pledged to "work towards maintaining peace and stability" in relations with China, but emphasised it must reflect public will.

Jubilant supporters expressed their faith in Tsai as she later addressed the crowds, promising to be a strong leader.

"I'm very confident - we were cheated by Ma's government for so long," said Jimmy Lai, 45.

Washington, Taiwan's main ally, congratulated Tsai on the victory and said the United States maintained a "profound interest" in peace between Taiwan and China.

White House spokesman Myles Caggins said the United States congratulated the people of Taiwan for demonstrating the strength of their "robust" democratic system.

"We maintain a profound interest in the continuation of cross-Strait peace and stability," he said. "We look forward to working with the new president and leaders from both parties to further strengthen the unofficial relationship between the United States and the people (of) Taiwan."

British Foreign Secretary Philip Hammond also congratulated Tsai, saying he hoped Taiwan and China would "continue their dialogue to resolve differences and maintain the recent trend of constructive relations".

Tsai remains president-elect until she takes office on May 20.

She secured 56.12 percent of the vote, according to the Central Election Commission, with the KMT's Eric Chu on 31.04 percent. It was by far the biggest mandate ever won by a DPP president.

Chu called the defeat "an unprecedented drastic change for the KMT" as the party also lost control of the legislature for the first time.

"This is a stunning result that completely overturns the history of Taiwanese elections. Never before has the DPP got anywhere near such a level in national elections," said Jonathan Sullivan, professor of Contemporary Chinese Studies at the University of Nottingham.

Analysts said ties with Beijing would inevitably cool as China watches Tsai closely.

"Cross-strait ties will be slowing in the near term as Beijing considers her to be independence-leaning. Future developments will depend on her actions," said Li Fei of the Taiwan Research Institute of China's Xiamen University.

Tsai disappeared from China's most popular social network following her victory, with censors working swiftly to block searches for "Tsai Ing-wen" and "Taiwan elections" on the Twitter-like Weibo network.

Ma had overseen a dramatic rapprochement with China since coming to power in 2008, culminating in a summit with Chinese President Xi Jinping in November.

Yet despite more than 20 deals and a tourist boom, closer ties have exacerbated fears that China is eroding Taiwan's sovereignty by making it economically dependent.

Low salaries and high housing prices are also riling voters who feel they have not benefited from the warming relations.

Beijing has warned it will not deal with any leader who does not recognise the "one China" principle, part of a tacit agreement between Beijing and the KMT known as the "1992 consensus" which is the bedrock of the rapprochement.

The DPP has never recognised the consensus.

Tsai has been thrust into one of Asia's toughest and most dangerous jobs, with China pointing hundreds of missiles at the island it claims, decades after the losing Nationalists fled from Mao Zedong's Communists to Taiwan in the Chinese civil war in 1949.

Tsai will have to balance the superpower interests of China, which is also Taiwan's largest trading partner, and the United States with those of her freewheeling, democratic home.