In Warsaw, demonstrators including former President Bronisław Komorowski gathered next to parliament. Some brandished banners with slogans including: "Independent courts - the right of every citizen."
The protests, held under the umbrella slogan "Judges today – you tomorrow", were organised by a number of groups including judges’ and prosecutors’ associations and the Helsinki Foundation for Human Rights.
Organisers said earlier that protests would also be held outside the Polish embassies in Brussels and London and the Polish consulate in New York, state news agency PAP reported.
Poland's governing Law and Justice (PiS) party, which came to power in late 2015 and won a second term in power in October, has argued that broad changes are needed to reform an inefficient and sometimes corrupt judicial system marred by communist holdovers.
The changes have triggered a series of clashes between Warsaw and Brussels.
Poland’s prime minister said earlier this year that some of the legal changes made by his conservative government have met with criticism abroad because they are not understood in Western Europe.
Poland Supreme Court warned on Tuesday that the country may eventually have to leave the European Union as a result of plans by the ruling conservatives under which judges could be fired for questioning the legitimacy of the reforms.
Under draft legislation put forward by deputies from the ruling Law and Justice party (PiS), judges could face penalties for challenging the appointment of those of their peers who took up their posts after the conservatives came to power, and for behavior deemed to be “political”.
Prime Minister Mateusz Morawiecki has warned that if judges question the appointment or verdicts of other judges, the Polish justice system could be engulfed by chaos.
The proposed new rules were expected to be debated by parliament on Thursday.
PiS politicians have said that rules similar to its new proposals exist in other countries, France in particular.
More to follow