Despite surviving an impeachment scare, Arroyo remains under pressure as the opposition refuses to drop allegations that she cheated in last year's presidential polls.
For the week to October 21, the composite index fell 30.64 points or 1.57 percent to 1,922.50 points.
Average daily volume rose to 371.9 million shares while value slipped to 793.88 million pesos (14.27 million dollars) from 317.55 million shares and 1.014 billion pesos last week.
"Market moves are dictated by political uncertainties and the market is digesting that uncertainty right now," said Nestor Aguila of DA Market Securities.
He cited the repeated holding of protest rallies by leftist and opposition groups as part of their continued effort to oust Arroyo.
Arroyo has taken a harsher stance against the rallies, with police using water cannons to disperse protestors who try to march on the presidential palace.
Aguila said that if the index falls below the 1,900-point level, then it would likely find initial support at the 1,892-point level.