He was speaking about changes that to the elected presidency when he suddenly went silent, and his face became contorted.
The next instant, he appeared to have lost his balance.
Fortunately, the PM remained conscious throughout.
The incident took place at about 9.20pm.
Aides and a medical team rushed to attend to Mr Lee.
The Prime Minister’s Office says that PM Lee took ill midway through his speech, and his condition has been assessed to be “not serious”.
The National Day Rally was suspended following the incident.
At around 9.50pm, Deputy Prime Minister Teo Chee Hean announced that PM Lee said he will return to conclude his speech.
On the sidelines of the rally, Deputy Prime Minister Tharman Shanmugaratnam told media at about 10.15pm that PM Lee is “perfectly normal”.
“I know it’s a very anxious moment, but doctors have assessed that he’s fine and it’s not a serious situation.”
PM Lee had been speaking since 6.40pm today.
He first spoke in Malay, then Mandarin, before starting his rally speech in English at 8pm.
The audience at ITE College Central, and those watching at home on TV, were understandably nervous as this comes 3 months after Finance Minister Heng Swee Keat’s collapse during a Cabinet meeting.
Mr Heng suffered a stroke and was rushed to Tan Tock Seng Hospital where doctors performed a surgery on his brain to close an aneurysm.
The operation was performed successfully and Mr Heng has been given time off official duties to recuperate.