The death of Singapore's first prime minister at the age of 91 has unleashed a national outpouring of grief.

Lee Kuan Yew's credited as the architect of the nation's rapid ascent from British colonial backwater to global trade and financial centre.

Mourners began gathering early to pay tribute outside the hospital where he died. 

The government's called a week of national mourning until Lee's funeral on Sunday.

His family's holding a private wake over the next two days.

The body will lie in state from Wednesday until Saturday.