The Haci Özbek mosque, according to the inscriptive plaque (kitabe) above a window, was built by Haci Özbek bin Muhammed in 734 A.H., three years following the conquest of Iznik by Orhan I. It is a single-unit mosque composed of a square hall crowned with a dome eight meters in diameter. The drum of the dome is dodecagonal and adorned with band of triangular planes on the interior. The three-bay portico preceding the hall to the west was torn down in 1940 to allow for road expansion. Carried on columns with Byzantine capitals, this portico was roofed with a barrel vault to the south and a mirror vault on the north. In its place, an enclosed portico was added to the northern side of the building in 1959. A minaret was never built. Ornamental details of the interior have been lost under layers of plaster. The construction of the mosque is brick and rubble stone, with saw-toothed brick cornices at the top of walls and terra-cotta tiles on the brick dome.
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