Zajączek was imprisoned in the
Poltava fortress until January 1814, and then remanded to
Białystok. After that, in July 1814, he was returned to Warsaw. There he was assigned to the post of a deputy president of a
Military Commission, reorganizing the former Napoleonic Army of the Duchy of Warsaw into a new, Russian-controlled
Army of the Congress Poland. Russian tsar
Alexander I, also newly crowned King of Poland, gave Zajączek on 3 December 1815 the position of the first
Namestnik of Kingdom of Poland (tsar's deputized ruler of the territory, a position similar to that of a
viceroy). This nomination surprised many, including Zajączek himself, who is said to have at first refused this position, but in the end accepted it after Alexander phrased his request as a direct order. Zajączek's years as a namestnik have garnered mixed reviews, both among his contemporaries, as well as among later historians. He was actively interested in economic development of Polish lands, and contributed to the
industrialization and
urbanization of the Kingdom's lands; he was also one of the founders of the
University of Warsaw. He has been criticized, however, for being inexperienced for such a major civil leader position, too servile to the wishes of his patron,
Grand Duke Constantine Pavlovich and he often disregarded the
Constitution of the Congress Poland, which contributed to a slow but steady erosion of the country's freedoms and autonomy. Deferential towards the Russian authorities, he readily let Duke Constantine and Alexander's commissar,
Nikolay Nikolayevich Novosiltsev, neither of whose positions were envisioned in the Constitution, become the
de facto rulers of the Kingdom. In recognition of Zajączek's services, Aleksander I bestowed upon him the title of '
Prince of Poland' on 27 April 1818. In the spring of 1826 Zajączek fell ill, steadily becoming weaker. On 25 July that year he lost consciousness, and died in the morning of 28 July at
Pałac Namiestnikowski (Viceroy's Palace), nowadays a seat of the
President of Poland. His funeral lasted from 2 to 5 August. He was buried at the
Church of the Holy Heart of Jesus in
Opatówek, where he had a manor on the lands received from Napoleon. His heart was placed in the
Bernardine Church in Warsaw, and his insides were buried at the
Powązki Cemetery, also in Warsaw. Zajączek and his wife had no children; Aleksandra outlived her husband by nineteen years. ==Honours and awards==