A sea of yellow raincoats filled the square and streets around the office of President Chen Shui bian on Saturday as demonstrators defied rain to demand a recount for last Saturday's poll, which Chen won by just 30,000 votes out of more than 13 million cast.
Dressed in black in a demonstration of mourning and standing in front of a huge sign reading "Democracy is Dead", Lien told the crowd he had counted more than 1,000 irregularities in the island's third ever direct presidential vote.
"Today so many people gather here to demand a fair election. This is the most basic value of democracy," said Lien, leader of the Nationalist Party who also lost to Chen in 2000.
"Our common demands are very simple: immediate recount. I want to ask Mr Chen Shui-bian: what are you afraid of? Stand up!"
"Stand up! Stand up!" the shouted back the crowd, estimated by police at nearly half a million.
About half a million people
took part in the demonstrations
Calls for inquiry
The Nationalist Party leader called for an independent inquiry into an election-eve assassination attempt that lightly wounded Chen and created a significant sympathy vote.
"We must be prepared for a long battle," Lien said. "We will not end our struggle until our goal is achieved. We must let the world see the power of the people."
The rivalry between Chen and Lien and the extent of support for the challenger threaten a prolonged crisis that could paralyse policy-making in one of Asia's most vibrant economies.
Chen does not have a majority in parliament, making it difficult for his policies to be passed into law.
Beijing said on Friday it would not sit idly by if the protests spiralled out of control.
China is anxious to prevent pro-independence activists who back Chen's policy of greater sovereignty from using the turmoil to promote their cause.
"Say no to Bulletgate scandal," read one placard held up by the crowd. "Say no to the dirty election," read another raised near a huge plaster replica of the Statue of Liberty.
Lien urged the crowd to go home, but warned Chen that his supporters would return in force to launch a new protest in early April if their demands over the disputed election were not met.
He said he would persist until the president's 20 May inauguration. The crowd then began to disperse.
An extra 5,000 police were deployed to maintain order and ranks of helmeted officers in full riot gear stood in rows behind barbed-wire barricades in front of the presidential palace.
Lien Chan (L) is demanding a
recount of the ballots
"In the years we have left we try to do something for our country," said veteran soldier Wang Feng, 78. "We demand a new election, but I don't know if that will happen."
China's Communist leadership issued its most strongly worded statement since the election, but stopped short of threatening action.
Beijing regularly threatens to use force to recover an island it claims as a renegade province, but was unlikely to follow up its latest angry words with action, analysts said.
Taiwan hit back, telling Beijing to mind its own business.
The Republic of China, the island's official name since it split from the mainland after the Nationalists fled there after losing a civil war to the communists in 1949, was capable of
dealing with internal dissent, it said.
On Friday, angry Lien supporters stormed the election commission office, smashing windows, throwing stones and eggs and scuffling with riot police, just before the commission formally declared Chen had been re-elected.
Analysts say Beijing, which regards Chen and his pro-independence ambitions with grave suspicion, clearly would have preferred a victory by Lien, who espouses a more conciliatory policy towards China.