The Supreme Court heard arguments on that provision of the law and another one that allows the government to collect data on the origin and destination of electronic messages, the Philippine Daily Inquirer reported.
The Cybercrime Protection Act was adopted last year. The court has stayed enforcement of the law until Feb. 5 and has to decide whether to extend an injunction and whether to find parts of the law unconstitutional.
Associate Justice Roberto Abad said the provision on libel would almost certainly have a chilling effect on speech. Abad compared liking or re-tweeting a libelous post to showing a friend a libelous statement posted on a bulletin board.
"It will make me now reluctant to express my view," he said.
Solicitor General Francis Jardeleza suggested Abad's comparison is flawed because someone who points out a statement on a bulletin board is not republishing it.
"Defamation is defamation whether we communicate through megaphones, letters, person to person, tweets, Facebook or e-mail," Jardeleza said.
Government lawyers called the provision on collecting Internet traffic data "barely constitutional." The law bars the government from collecting information on the contents of email.