Riot police prevented a protest in the Algerian capital Tuesday against state energy giant Sonatrach's controversial exploitation of shale gas in the southern desert.

Sonatrach announced plans last month to invest at least $70 billion (62 billion euros) over the next 20 years to exploit the gas despite huge public opposition in the In Salah area of the central Sahara, where successful test drilling was announced in December.

Schools, businesses and public offices in the region closed over concerns about possible environmental consequences, and demonstrations have been taking place regularly for two months.

On Tuesday morning, leaders of the opposition and supporters tried to gather in central Algiers, carrying placards that read "No to shale gas" and "We are all In Salah."

Police forcefully dispersed them and made a number of arrests.

Abdelkader Moukri, chairman of the Islamist party Movement of Society for Peace, later told a press conference: "Our action was a big success because we managed to break the silence."

He called for those arrested to be released.

And Ali Benflis, who ran unsuccessfully against incumbent Abdelaziz Bouteflika in last year's presidential election, said "we need a wide debate over shale gas that includes the people as well as institutions."

- 'Safeguard health, protect environment' -

On Monday, Energy Minister Youcef Yousfi announced the creation of an agency that will monitor prospecting.

And the president himself said Tuesday that while Algeria's hydrocarbon resources were a "gift from God" that should benefit present and future generations, the authorities should "scrupulously ensure that the health of the people is safeguarded and the environment protected."

Sonatrach aims to produce some 20 billion cubic metres (700 billion cubic feet) of shale gas per year from 200 drill sites.

To extract shale gas, a high-pressure mixture of water, sand and chemicals is blasted deep underground to release hydrocarbons trapped between layers of rock.

Environmentalists argue that the process -- known as fracking, or hydraulic fracturing technology -- may contaminate ground water and even cause small earthquakes.

Sonatrach chief Said Sahnoun has acknowledged that the company could have done more to allay public concerns but insisted they were unfounded.

"Perhaps we haven't communicated enough about the issue," he said last month. "The pilot well in In Salah is producing clean gas. The waste water is being managed."

Sahnoun said in early February that there was no question of halting exploration.

Algeria is the third-largest supplier of natural gas to Europe but its hydrocarbons sector has suffered in the past decade from a lack of foreign investment.

It is estimated to have the world's fourth-largest reserves of recoverable shale gas, after the United States, China and Argentina.

The sector was hit in January 2013, when Islamist radicals attacked the In Amenas gas complex, on the Libyan border, hundreds of kilometres (miles) east of In Salah.

Workers from Sonatrach, as well as British energy giant BP and Norway's Statoil, were taken hostage and 40 staff were killed during a four-day siege that ended with the army storming the complex.

Output plummeted following the attack, only returning to growth last October.