MANILA — A new Internet law that took effect in the Philippines on Wednesday could lead to imprisonment for common activities like sharing Facebook and Twitter posts, critics say.

The new law, the Cybercrime Prevention Act of 2012, which was signed by President Benigno S. Aquino III on Sept. 12, sets penalties for several computer-related crimes, including child pornography, identity theft, online fraud and illegally accessing computer networks.

But critics are concerned about the law’s provisions related to libel, which in the Philippines is a criminal offense that can result in imprisonment. Harry S. Roque, president of Media Defense South East Asia, an advocacy organization, said the law applied the existing legal definition of libel to the online activities of individuals, meaning that if a comment on Facebook or Twitter were deemed libelous, the writer of the item and those who shared it could be prosecuted.

“It exposes Internet users to prosecution,” said Mr. Roque, a law professor at the University of the Philippines. “You can be sued. You can be forced to post a bond. You will need to get a lawyer. You can imagine the mayhem this can cause.