Singapore Prime Minister Lee Hsien Loong had "taken ill" while speaking at the National Day rally today, but his office says his condition is "not serious".

"The medical team is attending to him right now. They assess that his condition is not serious," the Singapore Prime Minister's Office said in a statement.

He was feeling "unsteady because of prolonged standing, heat and dehydration", a statement released later on his Twitter account said.

"His heart is fine and he did not have a stroke," it said.

He resumed his speech about an hour after he went off stage.

Lee was welcomed back on stage with thunderous applause and a standing ovation.

"Thank you for waiting for me. I gave everyone a scare... I've never had so many doctors look at me all at once.

"They think I'm all right, but anyway, I'm going to get a full check-up after this but first let me get back to my speech," he said.

Update: PM was feeling unsteady because of prolonged standing, heat and dehydration. His heart is fine and he did not have a stroke. #ndrsg

— Lee Hsien Loong (@leehsienloong) August 21, 2016

Those following the live telecast of this speech saw him quivering and leaning to his right about three hours into his speech. 

The camera then panned to the audience and rustling was heard.

There was gasping among the audience and concerned expressions.

A picture published on The Straits Times showed him assisted off the stage.