
The 91-year-old, widely credited with transforming the city-state into one of Asia's wealthiest economies, has been in the Singapore General Hospital for more than six weeks and is being aided by mechanical ventilation, a form of life support.
"Mr. Lee Kuan Yew's condition has worsened," the office of his son, Prime Minister Lee Hsien Loong, said in a terse midday statement.
The government said earlier in the week that he was already critically ill.
Floral tributes and cards dedicated to Lee have started to pile up outside the hospital as Singaporeans showed affection for the authoritarian leader who modernised Singapore but is criticised by rights advocates for his iron-fisted rule.
"I cannot bear just sitting at home and listening to the news, not being able to do anything," retiree Phua Siew Lian, 88, said as she set a bouquet of sunflowers at a designated spot outside the hospital building where Lee is being cared for under tight security.
Lee was prime minister from 1959, when colonial ruler Britain granted Singapore self-rule, to 1990. He led Singapore to independence in 1965 after a brief and stormy union with Malaysia.
His deteriorating health has cast a pall over preparations for the city-state's 50th anniversary of independence on August 9, an event known as "SG50".
Some Singaporeans were also seen at the hospital saying quiet prayers after leaving flowers. The hospital, Singapore's biggest and oldest, is located just off the central business district.
The prime minister posted the latest health bulletin on his Facebook page and was immediately flooded with messages of support for him and his father.
He visited a community centre where residents offered tributes and encouragement for the patriarch to fight on.
"Dear Papa, Hope you get better!," he wrote on a banner.