Lee was hospitalized in early February with severe pneumonia. In the past week his condition has worsened to critically ill and the government has given daily updates. The latest, on Sunday, said he had "weakened further."
Lee commands immense respect among Singaporeans, who this year will celebrate the country's 50th independence anniversary. He led Singapore with an iron grip for more than three decades until 1990, and is credited with transforming the resource poor island into a wealthy bustling financial hub with low crime and almost zero corruption.
At Singapore General where the elder statesman is hospitalized, 26-year-old university graduate Kim Lee fought back tears as she stood near the growing pile of flowers and cards.
"I came to give him my medal. I just finished a running event this morning and I came to give it to him," she said. "To me he is more than a champion."
Singaporeans overseas also sent in messages of support. Among the cards was one from the Netherlands, signed "a faraway Singapore son." Another message hailed the "countless extraordinary things" Lee did for the tropical city-state and wished for his recovery.
State media reported that about 1,000 people had gathered at a community center in Lee's electorate, also leaving cards and gifts and writing messages on a giant banner.
Under Lee and his successors, including his son, current Prime Minister Lee Hsien Loong, Singapore was known around the world for its strict social order including a ban on chewing gum, restrictions on free speech and canings for crimes some countries would rule as minor. In recent years, it has become socially more liberal and the fragmented political opposition made gains in Singapore's last elections in 2011.
After stepping down as prime minister, Lee remained part of the Cabinet and an influential figure in Singapore and the region.
"We are very proud of Mr. Lee. He built Singapore into something so famous and so good," said David Kwok, 52. "Our people are very grateful to him."