The investigation will focus on whether criminal charges should be brought for "public propagating of fascism" during march on Poland's Independence Day.
During Turkish President Recep Tayyip Erdogan's one-day visit to Warsaw, aimed at boosting trade between the two countries, Polish President Andrzej Duda expressed support for Turkey's membership to the EU.
Since coming to power, the ruling party has not only increased government influence over the courts but has also brought prosecutors and state media under direct government control and introduced restrictions on public gatherings.
Poland's eurosceptic, nationalist government has rejected Brussels' objections as "blackmail" and as unjustified criticism, but has said Warsaw is open to talks to resolve the dispute.
Polish President Andrzej Duda on Monday announced that he will veto two of three bills reforming the country's judiciary system.
Tens of thousands of protesters gathered in Warsaw and cities across Poland for candle-lit vigils, chanting "Free Courts" and demanding that President Andrzej Duda, an ally of the ruling PiS party, veto the bill.
Polish President Andrzej Duda made a surprise compromise bid over controversial court reforms that aim to expand the justice minister's powers as thousands of protesters took to the streets over the proposed changes.
Polish President Andrzej Duda says the EU has not shown enough solidarity in understanding the situation of Central and Eastern European countries.
Jaroslaw Kaczynski blames European Council president Donald Tusk over Polish president's plane crash in 2010
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