The rally, held under the slogan "Everyone for Freedom", was organised by the Committee for the Defence of Democracy (KOD), which opposes sweeping reforms the conservative Law and Justice (PiS) party has pushed through since coming to power in October.
Saturday's demonstration was attended by former presidents Aleksander Kwaśniewski and Bronisław Komorowski.
Communist-era activists Władysław Frasyniuk, Krzysztof Łoziński and Danuta Kuroń, the widow of opposition leader Jacek Kuroń, also took part in the rally, alongside former Civic Platform Prime Minister Ewa Kopacz and the head of the Nowoczesna (Modern) opposition party, Ryszard Petru.
Solidarity icon Lech Wałęsa, meanwhile, did not take part in the Warsaw events.
Demonstrations, marches and pickets were also held in a number of other Polish cities as well as abroad throughout the day.
"We wish to celebrate, but also perhaps to express our concerns that the freedom of Poles may be somewhat threatened nowadays," the head of KOD, Mateusz Kijowski, said in the run-up to the event.
Saturday's rallies were the latest in a series of protests staged by KOD against changes carried out by Poland's conservative government.
Since taking power in last October's elections, the Law and Justice party has pushed through major overhauls of the country's top court, public media, the civil service and surveillance laws.
The reforms have sparked an outcry from critics who say the changes violate democratic values and principles - accusations which Law and Justice dismisses.
A protest organised by KOD in May drew tens of thousands of demonstrators, with the police estimating turnout at 45,000 but Warsaw City Hall claiming the number was 240,000.
The country's partially free elections held on 4 July 1989 saw a landslide victory for Solidarity trade union candidates, which ultimately led to the end of communism in Poland and triggered a domino effect in the region. (aba/pk)