With beads of sweat on his brow and his voice wavering, Prime Minister Lee Hsien Loong made a longer-than-usual pause right after stating that Singapore had worked hard over the decades to build racial harmony.

Some in the audience, like MP Zainal Sapari, thought Mr Lee had paused to collect his thoughts.

"In the past, when he talks about issues close to his heart, he can get quite emotional," he said. "It was only when he started swaying that I realised something was not right."

Others, like MP Alex Yam had noticed something amiss earlier, during Mr Lee's Chinese speech. "He usually does not trip over his words," said Mr Yam. "During the English speech, he stumbled over some words as well and he was reaching for his water more often. But it's been quite a hot day."

An audible gasp rose from the audience when the pause became a full stop, and Mr Lee's expression changed from one of animation to confusion as he clutched both sides of the rostrum to steady himself.

As some craned their necks to see what was happening, many covered their mouths in stunned silence.

A hush descended on the auditorium of ITE College Central as people tried to figure out what was happening, said MP Zaqy Mohamad.

From the first row, Deputy Prime Minister Teo Chee Hean, Defence Minister Ng Eng Hen and Acting Minister for Education (Schools) Ng Chee Meng rushed on stage.

Two security officers were also by Mr Lee's side in seconds.

There was a smattering of applause from the audience, as if giving hesitant encouragement to Mr Lee as he waved to the crowd while being led offstage. Mr Zainal said he heard Mr Lee say, "I'm sorry, I'm okay."

"As PM left, he actually raised his hand and I think people did not know how to react," he said.

Dr Ng then signalled to Minister of State for Education Janil Puthucheary and Minister of State for Health Lam Pin Min, and the two doctors went backstage to help.

An announcement of a break ensued and the Rally guests were invited to join the reception. But few felt like eating after what they had witnessed, said MP Baey Yam Keng. "People were anxious as they didn't know what had happened," he said.

Online and TV viewers took to social media to express their worry.

Many shared the sentiment of Facebook user Oli To: "Mr Lee, your health comes first. Please rest and recover fully. You had such an exhausting schedule in Singapore and overseas. Thankfully, it is nothing serious."

People felt somewhat reassured when at 9.31pm the Prime Minister's Office (PMO) announced on Mr Lee's Facebook page: "PM Lee took ill while speaking at the National Day Rally. The medical team is attending to him right now. They assess that his condition is not serious. PMO will update in due course."

A sense of normalcy returned at 9.51pm - almost half an hour after the break was called - when DPM Teo announced that Mr Lee was resting and would return to resume his speech. "PM Lee says please enjoy the reception, he's having a rest and he'd like to come back and greet all of you and conclude his speech."

Mr Baey said: "You could hear the relief when DPM Teo made the announcement."

Deputy Prime Minister Tharman Shanmugaratnam also told reporters at the reception that the scare was just "a little blip", and that doctors had assessed him to be fine. "He's perfectly normal... These things happen in life but he's strong enough to carry on. He's been having a very tough schedule recently, he's been standing for some time, so he's just feeling a little faint."

PMO confirmed this in a statement at about 10.28pm, saying Mr Lee had been feeling "unsteady because of prolonged standing, heat and dehydration". It added: "His heart is fine and he did not have a stroke."

At 10.40pm, Mr Lee strode on stage to a standing ovation, thanked the audience for waiting and shrugged off the incident with a smile. "The last time I did this, I was on the parade square in Safti (the military's officer training institute) and fainted," he said. "I've never had so many doctors look at me all at once. They think I'm all right, but I'm going to have a full check-up after this."

In a Twitter reply at 2am to Malaysian Prime Minister Najib Razak, Mr Lee said: "Thanks for your good wishes. My doctors tell me I should be OK."

•Additional reporting by Joanna Seow