Thousands of protesters are marching in Spanish cities to express their opposition to a proposed law that would set hefty fines for offenses like demonstrating outside parliament buildings or strategic installations.

Parliament approved the Public Security Law last month and it's expected to become official in February if passed by the government-controlled Senate.

Protesters with tape over their mouths and carrying banners calling the measures a "gagging law" gathered Sunday near Spain's parliament under heavy police surveillance.

The bill is heavily criticized by opposition parties and human rights groups as an attempt by the government to muzzle protests over its handling of Spain's financial crisis.

The law would allow fines of up to 600,000 euros ($745,000) for individuals demonstrating outside key buildings if they are deemed to breach the peace.