Pressure grew Wednesday for Philippine President Gloria Macapagal Arroyo (search) to quit as her opponents staged the biggest rally against her so far. The security forces went on full alert amid fears of terror attacks and swirling coup rumors.

Police estimated at least 40,000 people had gathered by late afternoon. The number was quadruple the previous largest anti-Arroyo rally but still a fraction of the giant "people power" rallies that ousted two Philippine presidents in the last 19 years.

Several people carried a huge banner reading "Goodbye Arroyo," as protest supporters rained white confetti on the crowd from nearby buildings.

Wilson Fortaleza (search) of the left-wing Sanlakas Party (search) called the protest "a preview for a bigger storm" of discontent to come.

"We will not stop marching until she steps down," said opposition Sen. Panfilo Lacson. "She must now think of the interest of the country."

Environment Secretary Mike Defensor, one of Arroyo's closest advisers, told foreign journalists that Arroyo "will never resign." He said the crisis over the president's alleged election fraud is "reversible" and that public trust can be restored.

He admitted, however, that the current situation is potentially explosive, calling it "just one notch short of getting violent."

The rally in Manila's Makati district was seen as a test of public sentiment and an indicator of whether the opposition can muster enough support to oust her in a "people power" revolt similar to those that removed longtime dictator Ferdinand Marcos (search) in 1986 and Joseph Estrada (search), Arroyo's predecessor, in 2001.

On both occasions, the Roman Catholic Church and the powerful military withdrew their support from the presidents — but that hasn't happened so far this time.

The demonstration was aimed at uniting a broad spectrum of opposition groups and parties in the biggest show of force since the political crisis erupted last month over allegations that Arroyo rigged the May 2004 election and her family received illegal gambling payoffs. Arroyo has apologized for talking to an election official before she was declared winner but denied manipulating the ballot.

Adding to Arroyo's problems, Moody's Investors Service followed the lead of two other rating agencies and downgraded the country's credit outlook to negative from stable, citing possible fallout on the budget and external payments if the political turmoil persists.