Police and troops were on high alert for violence by the protesters, a possible coup attempt or even terror attacks. Security at the presidential palace was extra tight. Container vans blocked some roads, and soldiers in battle gear manned checkpoints. Navy and coastguard patrol boats and assault teams sealed a stretch of river beside the palace and set up patrols.
The protest in Manila's financial district was as much about glamour as politics, with a steady stream of singers, dancers and politicians taking the stage to entertain a broad spectrum of opposition groups and parties.
The biggest show of force since the political crisis erupted last month - over claims that Ms Arroyo rigged the May 2004 election and her family took illegal gambling payoffs - was seen as a test of public sentiment and an indicator of whether the opposition can muster enough support to oust her.
But while the turnout was quadruple that of the largest anti-Arroyo rally so far, it was still a fraction of the giant "people power" rallies that forced out the dictator Ferdinand Marcos in 1986 and Joseph Estrada, Ms Arroyo's predecessor, in 2001.
"We will not stop marching until she steps down," an opposition senator, Panfilo Lacson, said. "She must now think of the interest of the country."