Spain's government Friday approved controversial new fines for holding unauthorised demonstrations, sparking protests from civil rights groups.

Following warnings from judicial authorities, Madrid watered down measures from an earlier bill, but maintained tough penalties for unauthorised demos if they are deemed a security threat.

Critics have branded the reform repressive. But Interior Minister Jorge Fernandez Diaz insisted it would "guarantee greater security for citizens with greater legal security... and protect the rights and freedoms of citizens".

He spoke at a news conference after the conservative government approved the latest draft at a cabinet meeting on Friday. It now heads to parliament where the ruling Popular Party holds a strong majority.

The new text sets fines of up to 600,000 euros ($815,000) for unauthorised demonstrations in public buildings that pose a threat to people or disrupt public services.

It sets a fine of up to 30,000 euros for unauthorised demonstrations near the national or regional parliaments if they are deemed to pose a serious security risk.

Spain's recent years of recession and tough economic reforms have sparked mass street demonstrations. Some rallies near parliament in Madrid boiled over into clashes between protesters and police.

The new law also sets a fine of up to 30,000 euros or obstructing authorities in their work -- as many protesters have done in Spain while trying to stop bailiffs evicting poor homeowners.

The law also doubles to 600 euros the fine for climbing public buildings and monuments without permission -- as the environmental campaign group Greenpeace often does.

Greenpeace members protested against the reform on Friday by climbing up the sides of a 90-metre (295-foot) tower near Prime Minister Mariano Rajoy's offices.

They unfurled a banner reading: "No to the gag law."

The government said the law on demonstrations "aims only to sanction violent, aggressive or coercive actions".

"The law expressly protects the right to demonstrate," it asserted in a statement.

But a protest group campaigning against the reform, named We Are Not a Crime, vowed to demonstrate in central Madrid on Friday evening in anger at the proposals.

"The government is trying to create a false impression of insecurity in society to justify toughening the laws at the expense of curbing citizens' rights," it said in a statement.