Warsaw city hall said, the protest drew some 240,000 people, making it one of the largest demonstrations since the 1989 collapse of communism in Poland.
The mammoth protest came as an annual pro-EU parade merged with a demonstration called by a coalition of pro-democracy groups and opposition parties.
The huge mass of marchers shouted pro-EU and pro-democracy slogans as they inched through the sun-drenched city centre, brandishing a mix of red-and-white Polish flags along with blue and gold EU flags.
The protest comes amid a mounting political crisis in the central European heavyweight, triggered by changes the populist-oriented Law and Justice (PiS) government has made to the constitutional court.
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In December, it pushed through legislation to stack the court and modify its decision-making rules.
The court itself struck down the changes as unconstitutional in March, pitting it against the PiS majority government, which wasted no time to dismiss the ruling.
The resulting deadlock means the court is paralysed, leaving Poland without a fundamental check on government powers.
Markets have also reacted strongly to the controversy, hitting Poland's zloty currency and the Warsaw stock exchange.
Ultra-nationalist parties and sympathisers organised a counter-demonstration in the capital today, drawing around 2,500 people insisting that Poland's EU membership meant it was subject to the "diktat of Brussels".