BRUSSELS (AP) — The European Union (EU) on Thursday launched legal action against member country Poland over a contentious new law that the nationalist government claims is meant to combat Russian influence but which critics say could be used to persecute opposition politicians.

The European Commission said it believes that the new law “unduly interferes with the democratic process,” and that it “violates the principle of democracy,” and “rights to effective judicial protection.” The EU’s executive branch supervises respect of the bloc’s laws.

The law was passed in May, ahead of expected general elections in autumn, and allows for the creation of a committee to investigate Russian influence in Poland. Critics argue that it would have unconstitutional powers, including the capacity to exclude officials from public life for a decade.

It appears to have galvanised public support for the opposition. Over the weekend, around 500,000 people took part in a huge anti-government protest, according to organisers. Citizens travelled from across the country to voice their anger at officials who they say have eroded democratic norms and created fears that the nation is following Hungary and Turkey down the path to autocracy.

The protest was led by the main opposition leader, Donald Tusk, a former top EU official. It was possibly the largest demonstration in decades in Poland, although state television said there were no more than 150,000 people. The broadcaster, TVP, was accused of sorely underestimating the turnout.

The march was held on the anniversary of a crucial moment in Poland’s history, the partly free election on June 4, 1989, which paved the way for the end of communist rule. It took place about five months before the elections, in which the ruling Law and Justice party is fighting for a third term.

As a first step in its legal action, the EU commission has sent the government in Warsaw a “letter of formal notice” outlining its grievances. Poland has 21 days to respond to the letter, and after more exchanges, the government could face hefty fines if it fails to comply.

Brussels is concerned that the law contains a broad and unspecified definition of “Russian influence” and “activities.” It considers that the law “violates the principles of legality and non-retroactivity,” because it could exclude officials from office for a decade for behaviour that was legal in the past.

The move comes just two days after the EU’s top court, the European Court of Justice, confirmed that Poland has refused to comply with the bloc’s rules on judicial independence. The government has already been fined more than 500 million euros ($535 million) over that case but so far refuses to pay.

On Wednesday, Poland’s minister for EU affairs, Szymon Szynkowski vel Sęk, said that the government would “provide legal and factual arguments in this case after getting acquainted with the doubts of the European Commission.”

“Poland, as the largest country neighbouring Russia, has not only the full right but also the obligation to investigate these influences,” he said, according to state news agency PAP. “We would like the decisions made in this matter not to be based on media hype, but on facts, and we will present these facts.”