TAIPEI (AFP) -- Tens of thousands of Taiwanese protestors were expected to again fill the streets of Taipei on Friday in a bid to step up the pressure on embattled President Chen Shui-bian to step down.

Organizers of a round-the-clock anti-Chen protest, now in its seventh day, predicted that between 300,000 and 500,000 people could join the evening rally, set to begin at 7:00 P.M. (1100 GMT).

Demonstrators armed with flashlights were to encircle Chen's office and nearby residence before converging on a square outside Taipei's main railway station to vent their anger at Chen over a series of corruption scandals.

"We will hold flashlights to spark hope and light up the dim presidential office and residence," said veteran pro-democracy activist Shih Ming-teh, who launched the "Million Voices against Corruption, Chen Must Go" campaign.

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

Taipei police were expected to deploy some 3,800 riot police to the area, amid fears that the protest could result in clashes between pro-and anti-Chen campaigners.

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

"We are a peaceful force which represents the people power ... the DPP lacks legitimacy if it mobilizes its supporters," Shih said. "Chen should be responsible if there is any drop of blood shed."

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

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

"Some 1,000 tourist buses, 2,000 taxis and 3,000 cars are expected to circle the Taiwan Taoyuan International Airport and occupy nearby highways to paralyze its operations," said organizer Mike Jen, head of the National Alliance of Labor Unions said on Thursday.

He said this protest would continue until Chen quit.

Premier Su Tseng-chang called for calm ahead of Friday's rally, as well as a mass pro-Chen rally set to take place in Taipei on Saturday.

"There must be a peaceful beginning and peaceful ending ... for the activities, so that people's calm lives and hard work are not jeopardized," Su said.

Local newspapers reported that, if necessary, Chen and his family would be taken to the island's top military command center for protection.

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 Parliament 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.