Zimbabwe's President Robert Mugabe has signed into law an act enabling state security agents to monitor phone lines, mail and the Internet, a government notice published on Friday said.

Officials have said the new law is designed to protect national security and prevent crime, but human rights groups fear it will muzzle free speech under a crackdown on dissent.

In the government notice, Chief Secretary to the President and Cabinet Misheck Sibanda said Mugabe had agreed to the Interception of Communications Act, which was approved by both houses of Zimbabwe's parliament in June.

The law gives police and the departments of national security, defence intelligence and revenue powers to order the interception of communications and provides for the creation of a monitoring centre.

Postal, telecommunications and internet service providers will be required to ensure that their "systems are technically capable of supporting lawful interceptions at all times".

Critics have said the law is a government ploy to keep tabs on the opposition at a time when political tensions are mounting and Mugabe is deflecting growing criticism from Western powers.

Zimbabwe is suffering a severe economic crisis, marked by the world's highest inflation rate, 80 percent unemployment and persistent food, fuel and foreign currency shortages.

The southern African country, once viewed as a regional bread basket, cannot feed itself and faces severe shortages of basic consumer goods after a government-ordered price freeze in June that has emptied shop shelves.

Mugabe -- Zimbabwe's ruler since independence from Britain in 1980 -- denies controversial policies such as the seizure of white-owned farms to resettle landless blacks have ruined the economy, and blames Western sanctions for the economic turmoil.