Płock (plôtsk) [key], city (1993 est. pop. 126,900), Mazowieckie prov., E central Poland, a port on the Vistula River. Płock is a major oil-refining and petrochemical center. Other industries include food processing and the manufacture of farm machinery. Known in the 10th cent., it became a bishopric in 1075 and was the capital of the duchy of Mazovia from 1138 to 1351. The city passed in 1793 to Prussia and in 1815 to Russia. It reverted to Poland in 1921. Płock's 12th-century cathedral, containing tombs of Polish kings and dukes, was badly damaged during World War II.