Early on Saturday, police barged into the offices of The Daily Tribune, a newspaper critical of Arroyo, and confiscated editorial materials and a bundle of newspapers headlining the president's declaration.
The same day, Crispin Beltran, an opposition congressman, was arrested by the police at his home just outside Manila on sedition charges.
Arroyo declared the emergency on Friday to quell an alleged conspiracy by military officers and civilian allies coinciding with the 20th anniversary of the 25 February 1986 fall of dictator Ferdinand Marcos.
Demonstrations linked to the anniversary of the "People Power" revolt, which the opposition is using to demand Arroyo's resignation, have been banned.
Meanwhile, Fidel Ramos, a former president who stood by Arroyo during last year's failed impeachment attempt, joined Arroyo's critics, saying in a TV interview: "I was not only surprised, I was appalled and dismayed" by the declaration.
"If there is a plot of conspiracy, then an immediate investigation must take place to determine whether an actual crime was committed "
Ramos accused Arroyo of "killing the spirit" of the 1986 revolt.
He said he did not believe the declaration was justified because no actual coup attempt had taken place, and that Arroyo had overreacted.
Ramos, a military commander who withdrew his support for Marcos and was a major figure in Marcos' downfall, said: "If there is a plot of conspiracy, then an immediate investigation must take place to determine whether an actual crime was committed."
He did not call for Arroyo's resignation, as his predecessor, Corazon Aquino, has done.
Aquino, the icon of the 1986 revolution who restored democracy in the Philippines, led a street protest on Friday during which she urged Arroyo to step down.
While declaring the emergency in a televised statement, Arroyo said: "This is my warning against those who threaten the government: the whole weight of the law will fall on your treason."
Her chief of staff, Mike Defensor, said the declaration bans rallies, allows arrests without warrants, permits the president to call in the military to intervene, and lets her take over facilities - including media outlets - that may affect national security.