Leaders in China and Taiwan have used the 60th anniversary of the end of Japan's rule over Taiwan to back their differing views on the island's status.
Jia Qinglin said the return of Taiwan to China was "undeniable"
Taiwan's President Chen Shui-bian said the island was responsible for its own affairs.
"The greatest significance of Taiwan's restoration [from Japan's rule] is that Taiwanese have become masters of their own house," he was quoted as saying.
But China's rulers said the anniversary showed Taiwan was part of China.
"The legal status of Taiwan as an inalienable part of China's territory is beyond doubt, and can never be challenged," Politburo member Jia Qinglin told senior officials gathered to celebrate the anniversary.
Many Taiwanese see their island as a separate country. China sees it as part of its territory and has threatened to use force if Taiwan moved towards formal independence.
In the past China has shied away from marking the anniversary of Taiwan's liberation from Japanese rule.
Analysts say this is because Beijing did not want to complicate its claim that the Communist Red Army - rather than the Nationalists - defeated Japan.
Beijing is holding an exhibition to highlight Taiwan's anti-Japan war efforts
But this year, Beijing has taken a new approach, deciding to mark what the state news agency Xinhua has described as "the recovery of Taiwan from Japanese occupation".
Government officials gathered in the Great Hall of the People in Beijing to mark the anniversary.
China seeks "to encourage compatriots on both sides of the Taiwan Strait to firmly oppose the separatist forces," Jia Qinglin said.
"The situation is still very serious and complicated," Mr Jia said. "The Taiwan independence forces have not given up."
But Taiwanese President Chen was quoted as telling a group of Japanese and American academics in Taipei that the end of Japanese colonial rule in Taiwan "by no means signifies a return to China".
Taiwan was ceded to Japan in 1895.
But in 1945, following the end of World War II, it was forced to give the island up to the Nationalist government which was then in control of much of China.
Just four years later, the Communists took control of the mainland and the former Nationalist rulers fled to Taiwan.
Since then there has been a tense relationship between Beijing and Taipei.
The Communist authorities have even threatened to invade the island if it attempts to seek formal independence.