Tens of thousands of Spanish police officers and their supporters marched in Madrid on Saturday to protest against government plans to reform a controversial security law known by critics as the "gag law."

Critics of the Citizens Security Law passed by the previous conservative government in 2015 have for years said that it gave too much power to security forces to the detriment of civil liberties.

Powerful police unions, however, say that the proposed changes to the law will make their job more difficult.

Eugenio Zambrano, union leader from the Central Sindical Independiente y de Funcionarios (CSIF) said, "(this law) give us a number of uncertainties to work on a technical level and also for our physical security, but the citizens are the more affected."

A new version of the law sponsored by the small Basque Nationalist Party (PNV) recently won the support of Spain's governing left-wing coalition.

Amnesty International and Spain's Ombudsman Office (Defensor del Pueblo) have also called for the law to be altered.

The proposed law could still undergo changes during negotiations in the Parliament's lower chamber, but as it now stands it would eliminate some of the most contentious parts of the current law.

These include the article that banned holding protests in the immediate vicinity of Congress or Senate buildings and the article that allowed border guards to push back migrants who had crossed the frontier.

A new tweak that is supported by the government is the allowance for spontaneous protests that now commonly arise from quick organisation of a march, for example, to respond to a case of gender violence.

Currently, organisers of protests or marches should tell authorities beforehand.

Police unions are against other planned modifications, above all one to remove a requirement for citizens to request permission from authorities before filming and publishing video of officers at work.

Last year Spain's Constitutional Court ruled that such a requirement for previous approval was unconstitutional.

But police fear that could make their officers easy to identify and thus put them at risk of reprisals.

The proponents of the law deny this, promising that the new law is about striking a better balance between liberty and safety.

Antonio, a Civil Guard officer participating in the march, said this law is going to make things easy for criminals, "citizens will be less protected and it is going to be easy for the criminals."

Right-wing opposition parties backed the police protestors.

Both the far-right Vox party and the Popular Party that passed the original security law while in power sent their leaders to the rally.