Many Facebook and Twitter users and the portals of media organizations in the Philippines have replaced their profile pictures with black screens to protest the law. Hackers also defaced several government websites in protest.
Renato Reyes, secretary general of the left-wing New Patriotic Alliance, another petitioner, said the court's order was "a major victory for freedom and civil liberties."
Aquino has supported the online libel provision, saying people should be held responsible for their statements. But he has also said he is open to lowering the penalties.
Several legislators, including some who approved the law, have said they will try to amend the law to address civil-rights concerns.