Media groups and Filipinos have stepped up calls to repeal a tough new law that targets cybercrime.
Activists fear the legislation, which took effect on Wednesday, will be used to suppress online freedoms in the Southeast Asian nation, AP news service reported.
The Cybercrime Prevention Act went through despite last-minute petitions to the Supreme Court to stop it.
The justices said they will take up the issue next week.
The law is envisioned as a measure against hacking, identity theft, spamming, cybersex and online child pornography.
But citizens and groups who protested on social networking sites, blogs and out in the streets fear
politicians will use it to silence critics.
The law contains a provision that says libel - which is already punishable by up to six years in prison - is also a cybercrime.
It doubles cumulative penalties for online offenses and allows government agencies to search, seize
and destroy computer data deemed libelous.
Human rights and media groups have unsuccessfully campaigned for years to downgrade libel from a criminal to a civil offense, saying politicians often use the law to harass journalists and other critics.
The Department of Justice also now has the power to close down websites and monitor online activities, such as email or messaging, without a warrant, according to AFP news service.
Former President Gloria Macapagal Arroyo's husband sued 46 investigative journalists and publishers in more than 50 libel cases from 2003 to 2007 but later dropped them in a "gesture of peace".
The journalists wrote stories alleging Jose Miguel "Mike" Arroyo was corrupt, which he denied.
He is now facing two corruption cases linked to an overpriced government deal and the sale of secondhand helicopters to police.
The National Union of Journalists of the Philippines said that the new law's criminal libel provision "and the insidious way it was inserted during the bicameral deliberation - without benefit of public consultation - are direct strikes at the rights to free expression and press freedom".
Journalist Alexander Adonis, one of seven petitioners against the law who himself was jailed on libel charges from 2007 to 2009, argued that the law is unconstitutional and its provisions "so vague, so overbroad that these can be applied arbitrarily on all users of social media".
"In the context of the cyberworld, 'libel' is very difficult to determine since there are many actors in the cyberworld," including the blogger, the blog service provider, the Internet service provider, the person who comments on the blog and the person who posts a link to the blog site, he wrote.
Presidential spokesman Edwin Lacierda assured the public Wednesday that the constitution "is clear and uncompromising in the civil liberties it guarantees all our people".
President Benigno Aquino III's administration has not pursued any libel cases since he took office in 2010.
Lacierda criticised hackers who defaced many government websites in support of the movement against the cybercrime law, saying they engaged in online vandalism.
Many Facebook and Twitter users in the Philippines and the portals of the main media organisations have replaced their profile pictures with black screens as a protest against the law.
On Twitter, the hashtag #notocybercrimelaw became the top trend on the microblogging site in the Philippines on Wednesday, according to two trend mapping websites.
One tweet from @PmlFlrs read: "They signed and implemented this law because government officials can't