They marched a 5.5-kilometer (3.4-mile) route circling Chen's office and nearby residence amid downpours caused by an approaching typhoon.

Protesters give the thumbs down sign during a demonstration in Taipei. Tens of thousands of Taiwanese protesters took to Taipei's streets in a bid to step up the pressure on embattled President Chen Shui-bian to resign over alleged corruption.[AFP]

Many dressed in red, waved flashlights, chanted slogans demanding Chen's resignation and gesticulated the thumbs-down sign to vent their anger at the president over a string of corruption scandals.

"Chen already lost the people's trust and he was unfit to rule anymore. He must quit to save Taiwan," said Michael Huang, 47, who marched with his wife and two children Friday.

The Huang family and some fellow protesters trod on an effigy of Chen in a show of anger.

"We must succeed to create a clean and corruption-free Taiwan," said pro-democracy veteran Shih Ming-teh, who launched the "Million Voices against Corruption, Chen Must Go" campaign.

"We stand here today in heavy rains because our leader made grave mistakes ... A Bian steps down," Shih told the crowd when he began the rally, referring to the president using a nickname.

The march started at around 6:00 pm (1000 GMT), one hour early, because a larger-than-expected crowd gathered outside the presidential office.

It was expected to converge on a square outside Taipei's main railway station by the end of the march.

Shih's camp, which organized a round-the-clock protest now in its seventh day, estimated a turnout of nearly 300,000 as of 6:30 pm.

Chen's office said his schedule was unaffected by the protest. It rejected media reports that the president and his family were to be taken to the Taiwan's top military command centre for protection.

Some 3,800 riot police were expected to be deployed amid fears that the protest could trigger clashes between pro- and anti-Chen campaigners.

Shih, a former chairman of Chen's Democratic Progressive Party (DPP), has said he is prepared to continue his campaign until the middle of 2008, when the president's term in office expires.

He warned the DPP against mobilizing its supporters to interfere with the march, saying Chen would bear responsibility for any violence.

The warning came as the anti-Chen movement looked set to spread after unions vowed to create havoc at the main airport and on highways.

Organisers said thousands would stage a sit-in protest at the entrance of the airport no later than October 10, Taiwan's National Day.

Premier Su Tseng-chang and Taipei mayor Ma Ying-jeou, also head of the main opposition Kuomintang, both called for calm ahead of the march and a mass pro-Chen rally, which is set to take place in Taipei on Saturday.

Chen's troubles started in May after his son-in-law Chao Chien-ming was detained and later indicted on suspicion of insider trading and taking bribes. Chen has publicly apologized for Chao's actions but said he would not resign.

Prosecutors then began looking at whether Chen had misused funds intended for national affairs and questioned him last month.

In June he survived an unprecedented parliamentary vote to topple him, after the move failed to win the backing of two-thirds of all lawmakers. If passed, it would have triggered a national referendum on Chen's future.

His wife Wu Shu-chen is also under investigation for allegedly accepting department store gift certificates in exchange for lobbying efforts.

The independence-minded Chen, first elected in 2000 after more than 50 years of nationalist Kuomintang rule, narrowly won re-election in 2004 for a second and final term.