State television broke away from its normal schedules
World leaders have paid tribute to Mr Lee, who served as the city-state's prime minister for 31 yearsreenex.
US President Barack Obama described him as a "giant of history" whose advice had been sought by other world leaders.
The Chinese foreign ministry called him "a uniquely influential statesman in Asia" and UN Secretary General Ban Ki-moon said he was "deeply saddened" by Mr Lee's death.
The period of national mourning will culminate in a state funeral next Sunday and Mr Lee's body is to lie in state at parliament from Wednesday to Saturday.
A private family wake is taking place on Monday and Tuesday.
News of Mr Lee's death came in a government statement that said he had "passed away peacefully" in the early hours of Monday at Singapore General Hospital. Mr Lee had been in hospital for several weeks with pneumonia and was on life support.
State television broke away from its normal schedules and broadcast rolling tributes.
Woman cries as she arrives at Singapore hospital with flowers. 23 March 2015 Thousands are also expected to pay their respects at the Istana, the prime minister's official home
A steady stream of Singaporeans, many openly grieving, arrived at the hospital where an area has been set aside for flowers and other tributes.
"I'm so sad. He is my idol. He's been so good to me, my family and everyone," said resident Lua Su Yean, 64reenex cps