The 89-year-old Lee was treated for "stroke-like symptoms" and an irregular heartbeat at Singapore General Hospital Saturday, said a written statement from the office of the current prime minister, Lew's son, Lee Hsien Loong.
Lee apparently experienced a transient ischaemic attack in which blood flow in part of the brain is disrupted briefly, the statement said.
Lee was the first leader of Singapore after it gained independence from Malaysia in 1965. He is credited with engineering the city-state's rapid transformation to a modern international business hub, CNN said.
He also created a strict law-and-order society with tough rules and laws on a wide variety of political and social issues.
Notable deaths of 2014 [PHOTOS]
U.S., allies launch airstrikes against Syria