Thousands took to the streets in support of TVN24 and free media

Parliament is scheduled to vote on the legislation on Wednesday.

The vote to tighten rules on foreign ownership of Polish media threatens to sour relations with Washington and deepen concern in the European Union over democratic standards in the bloc's east. Issues such as judicial independence and LGBT rights have already brought Poland and Hungary into conflict with Brussels.

In Warsaw, one of around 90 towns and cities where protests were organised, people brandished placards with slogans such as "Free Media, Free People, Free Poland".

"This government is terribly weak, disorganised and cowardly," said Polish opposition leader Donald Tusk at a national convention of his Civic Platform party. "Their only response to Polish problems is gagging independent media and giving raises for its politicians," he added.

TVN24 footage showed protesters on the roof of the Culture Ministry in Warsaw with banners reading "Free Media" and "Poland Free of Facism".

The amendment to the Broadcasting Act would strengthen a ban on non-European firms controlling Polish broadcasters.

"Would we like Polish media... to be acquired without any regulations by anyone from around the world without any obstacles?" Prime Minister Mateusz Morawiecki asked reporters earlier.

While Morawiecki's Law and Justice (PiS) party says it wants to stop countries such as Russia or China taking control of Polish broadcasters, critics say it aims to gag TVN24, Poland's most popular news channel, whose licence expires on September 26.

TVN24's parent, TVN, is owned by the U.S.-based media group Discovery Inc. via a firm registered in the Netherlands, to get around a ban on non-European firms owning more than 49% of Polish media companies.

TVN has called the bill, which would close that loophole, an attempt to limit media freedom.

PiS has long argued that foreign media groups distort public debate in Poland in a way that harms Polish interests.

But the bill risks upsetting an important ally.

TVN, estimated to be worth over $1 billion, is the biggest American investment in Poland. U.S. State Department Counselor Derek Chollet has said future American investments could be jeopardised if TVN24's licence is not renewed.

Critics say the government is adopting tactics tried and tested by Hungarian Prime Minister Viktor Orban, a close ideological ally of PiS.

Hungarian public broadcasters have largely become mouthpieces for the government, while several other media outlets have been shut or taken over by government-friendly interests.

Critics say that since PiS came to power, Poland's public broadcaster, TVP, has become an outlet for government propaganda.