CARACAS, Venezuela – Some Venezuelan television channels began altering their programs yesterday, citing fears of penalties under a new law restricting violence and sexual content over the airwaves.

The law, which took effect yesterday, limits broadcasts deemed to be obscene or violent and details a range of offenses for which the government may fine noncompliant press organizations.

The private TV channel Globovision blocked out photographs of street violence with white space when it displayed the day’s newspapers, filled with coverage of Wednesday riots which police said left at least 56 injured.

“We cannot show the images,” said Carlos Acosta, who hosts the morning news program “Front Page,” as cameras focused on several Caracas newspapers.

President Chavez signed the Law for Social Responsibility in Radio and Television on Tuesday night, following its approval by legislators last month.

Critics say the law threatens press freedoms, and have dubbed it the “gag law.”

But Mr. Chavez and his supporters say they are committed to freedom of expression. They say the changes will ensure more responsible programming and television that is suitable for children and adolescents.

Nevertheless, organizations such as the Inter American Commission on Human Rights and Human Rights Watch have expressed concern.

A state Department spokesman, Adam Ereli, said yesterday that America is “deeply troubled” by “threats to freedom of expression” posed by the law.

The law distinguishes between news and opinion programming. It also bans “vulgar” language, images of sex, and “psychological” or physical violence from 7 a.m. to 11 p.m.