The protesters were demanding that the government step down and call for an early election.
Among the killed are 14 civilians, including a Japanese TV cameraman and five soldiers.
"The injured are being treated in the General, Hua Chiew, Mission, Phra Mongkutkhao and other hospitals," the center of the Bangkok Metropolitan Administration said.
Thai Prime Minister Abhisit Vejjajiva expressed sorrow for the deaths in the clashes and promised to conduct an independent probe into the deaths.
Government spokesman Panitan Wattanayagorn said, "The Prime Minister's secretary is coordinating with protest leaders to solve the situation and would like protesters to stay put."