The mass is being attended by Polish President Andrzej Duda, his wife Agata Kornhauser Duda, Prime Minister Mateusz Morawiecki, the speakers of the lower and upper house of the Polish parliament, Marek Kuchcinski and Stanislaw Karczewski, and other state officials.
After the mass, the president will present state distinctions at the Royal Castle.
The main ceremonies will start at noon at Zamkowy Square.
The May 3 Constitution was passed on May 3, 1791, by the Great Sejm (grand parliament) of the Polish–Lithuanian Commonwealth and was Europe's first document of its kind, predating the famous French Constitution and following the American one by only four years.
It introduced a hereditary constitutional monarchy (in place of the free election of kings), religious tolerance and the division of power into the legislative, executive and judicial branches, among other provisions. (PAP)
mr/ej