Hundreds of Spaniards marched in Madrid and similar protests were called in other cities on Saturday against a government plan to toughen penalties for unauthorised demonstrations.

Blowing whistles and banging drums, several hundred marched to Madrid's central Puerta del Sol square. Organisers called for similar gatherings in about 20 other cities.

It was the latest in a string of protests against the proposed reform, dubbed the "Gagging Law" by its critics.

The bill increases possible jail terms for certain public order offences and sets fines of up to 600,000 euros ($683,000) for organising unauthorised demonstrations.

The government says the new legislation will improve public security and protect rights and freedoms.

It has been passed by the lower house of parliament and is due to be voted on by the Senate in the coming weeks.

Both houses are controlled by the conservative governing Popular Party, so it is likely to pass before a general election due in November.

Joining in the Madrid march was international environmental group Greenpeace, which often stages demonstrations and stunts in its campaigns.

Its activists pushed along a big model lion in a cage with a yellow gag over its mouth -- a replica of a statue that stands outside parliament in Madrid.

Spain saw a rise in protests after an economic crisis broke out in 2008 that has left millions of people out of work.

"The government remains set on silencing the voice of the people," said Miguel Angel Soto, a spokesman for Greenpeace Spain.

"They are gagging dissenting voices and want to fine citizens who do not resign themselves to what we are going through."