"We will not sit by watching should the post-election situation in Taiwan get out of control, leading to social turmoil, endangering the lives and property of our flesh-and-blood brothers and affecting stability across the Taiwan strait," Beijing's policy-making Taiwan Affairs Office on Friday said in a statement.
It reiterated that China was paying close attention to events on the island, which has been racked by protests and political paralysis since President Chen Shui-bian, who favours Taiwan independence, won re-election by the thinnest of margins on Saturday.
The opposition immediately contested the result and demanded a recount.
The Chinese statement gave no further details. Beijing regards Taiwan as a renegade province and has threatened to use military force to bring it back to the fold.
China's response came after angry supporters of defeated Taiwan presidential challenger Lien Chan stormed the Central Election Commission on Friday, but failed to stop it from formally declaring Chen the winner.
The president edged Saturday's poll by just 30,000 votes out of 13 million cast, one day after surviving an apparent assassination attempt.
The protesters at election headquarters in central Taipei threw rocks and eggs and scuffled with helmeted riot police carrying shields before storming into the building where commission members were meeting.
Anger has spilled onto Taiwan's
Two burning petrol bombs were found outside the headquarters of the opposition People First Party, said a spokesman for the party, which is allied with Lien's
Nationalists. No damage or injuries were reported.
China has not commented directly on the result of the election to avoid playing into Chen's hands, but it has played up the opposition protests in reports carried in domestic media.
Analysts said China, which regards Chen with deep suspicion, clearly would have preferred the more moderate Lien.
Beijing and Taipei have been rivals since their split at the end of a civil war in 1949, but trade, investment and tourism have blossomed since the late 1980s.
Shortly after the vote, Beijing condemned Chen for holding the island's first referendum in tandem with the presidential vote, but said only that it was closely monitoring post-election developments.
On Tuesday, the Foreign Ministry stressed that no matter who won, Taiwan belonged to China.