Cuba (search) tightened its controls over the Internet on Friday, prohibiting access over the low-cost government phone service most ordinary citizens have at home.

The move could affect hundreds, perhaps thousands, of Cubans who illegally access the Internet from their homes, using computers and Internet accounts they have borrowed or purchased on the black market.

Cuba's communist government already heavily controls access to the Internet. Cubans must have government permission to use the Web legally and most don't, although many can access international e-mail and a more limited government-controlled intranet at government jobs and schools.

Now Cubans will need additional approval to access via the nation's regular phone service. Since few Cubans are authorized to use the Internet from home -- only some doctors and key government officials -- the new law amounts to a crackdown on illegal users.

The law states that the move is necessary to "regulate dial-up access to Internet navigation service, adopting measures that help protect against the taking of passwords, malicious acts, and the fraudulent and unauthorized use of this service."

As for foreign firms and individuals, most are authorized to use the Internet in Cuba, usually via a more expensive telephone service charged in American dollars and already off limits to most Cubans.

E-net, the Internet service of the Cuban telephone company Etecsa (search), told customers in a letter Friday the new law would take effect late Saturday. It affects all other Internet service providers in Cuba as well.

E-net (search) is the largest of a handful of Internet providers in Cuba -- all of them heavily monitored and controlled by the government.

E-net customers who do not have the dollar phone service can keep accessing the Internet with the ordinary phone service with special cards sold at Etecsa offices, the letter says.