Ten thousand supporters gathered outside Chen's Taipei campaign headquarters, waving the green banners of his Democratic Progressive Party (DPP) and setting off a storm of firecrackers when news broke that Chen had taken an unbeatable lead.

"We won. It's by about 50,000 votes," said a senior DPP source who declined to be identified.

However, Nationalist Party opponent Lien Chan swiftly challenged the razor-thin result, describing the election as unfair and invalid and saying he would ask for a recount.

"We want to raise a motion to declare the election invalid," Lien, head of the Nationalist Party, told a cheering crowd of thousands at his campaign headquarters.

"We ask the central election commission to seize and seal all ballot boxes," he said, adding that he wanted a recount.

With more than half the vote counted in official results, Chen had taken a lead that made it almost impossible for his opponent to win, the Central Election Commission (CEC) said.