A group of journalists will launch a triple lawsuit at the European court of human rights this week in an attempt to force Spanish MPs to repeal a security law that cracks down on the right to assembly and freedom of expression.

The conservative People’s party government was severely criticised when it approved changes this year to public security legislation, nicknamed the “gag law” by critics.

The modifications set out strict guidelines on when and where protests can take place, stipulating fines of up to €600 (£436) for “disrespecting a police officer” and up to €600,000 for holding an unauthorised protest near key infrastructure such as transport hubs or telecoms installations.

The law also includes fines of up to €30,000 for disseminating images of police officers that “would endanger their safety or that of protected areas or put the success of an operation at risk”.

The lawsuits expected to be filed in the European court on Tuesday take aim at this clause. Backed by the platform Defender a Quien Defiende, or To Defend Those Who Defend, three groups of journalists representing media from across the country will file parallel lawsuits, arguing that the law encourages self-censorship and paves the way for diminished accountability of police forces in Spain.

Central to their case is a letter sent in September by the central government to the association that runs the news site Ahotsa.info regarding images it published showing two officers standing with San Fermín revellers outside a bar.

The letter warns the site to avoid “such incidents”, explaining that failure to do so could result in a fine of between €601 and €30,000 under the recently passed law. “The image is completely harmless, the police officers aren’t identifiable,” said Diego Boza, one of the lawyers behind the lawsuits. “But they are demanding that they censure future publications.”

As no fine was levied in the letter, the warning cannot be appealed against. But as the scope of the human rights court allows groups to challenge laws they believe violate their rights, the platform decided to bypass the Spanish courts and launch the lawsuits at European level.

“They are limiting the right to communicate information, in that police acts can’t be published even when the subjects aren’t identified,” said Boza.

It could take months before the court decides whether to hear the case. The lawsuits are timed to coincide with the Spanish general election on Sunday. “We want to make it a conversation topic in the elections,” said Boza. All of the main parties running against the People’s party have vowed to repeal the law if they win power.

The government has repeatedly defended the bill, insisting that it does not prohibit photos taken for informative purposes but merely those that endanger police or their family. “It’s a law for the 21st century,” Spain’s interior minister, Jorge Fernández Díaz, said this year. “It provides better guarantees for people’s security and more judicial security for people’s rights.”

Concerns about the law’s impact on journalists were raised in a report published by a group of international press freedom organisations this year. The International Press Institute said the “vague and disproportionate provisions in the public security law risk chilling news media and harming the Spanish public’s right to information on matters of public interest”.

Others have inadvertently found themselves caught up in this tangled web; in August a Spanish woman was fined €800 for posting online a photograph of a police car parked illegally in a bay reserved for disabled people.