Polish President Bronislaw Komorowski, Prime Minister Donald Tusk, former Presidents Lech Walesa and Aleksander Kwasniewski, and European Commission President Jose Manuel Barroso were among those gathered for the service presided over by Cardinal Kazimierz Nycz, the archbishop of Warsaw.
Mazowiecki's coffin was moved Saturday from the presidential palace to the Cathedral Basilica of St. John the Baptist for the funeral service, thenews.pl reported.
His final resting place will be a family grave in Laski, next to his wife Ewa.
Mazowiecki was a key negotiator for the union movement in 1989 and took the post of prime minister in September of that year with a vote of confidence by the country's first parliament.
He ran for president against Lech Walesa in 1990 but withdrew from the race after the first ballot.
He became special emissary for the United Nations to Bosnia and Herzegovina during the Balkans war and served as a member of the Polish lower house of parliament for the center-right Freedom Union.
He was a vocal supporter of an integrated Europe, pushing for Poland's membership in the European Union.