Singapore Prime Minister Lee Hsien Loong took ill on Sunday in the middle of his annual National Day Rally speech because of heat and dehydration, his office said.

"PM Lee was feeling unsteady because of prolonged standing, heat and dehydration. His heart is fine and he did not have stroke," said an update from the prime minister's office.

The rally was temporarily suspended at 9:20 pm after the premier paused and stumbled midway through his English speech. He had already given his Malay and Mandarin language speeches.

Deputy Prime Minister Teo Chee Hean later told the audience assembled at the rally venue that Lee would greet the crowd and carry on with his speech after taking a rest.

Lee "took ill while speaking at the National Day Rally. The medical team is attending to him right now," said an earlier statement from his office, adding that his condition was "not serious."

The Straits Times newspaper reported that Lee was escorted off the stage with the aid of Teo, Defence Minister Ng Eng Hen and Acting Education Minister Ng Chee Meng.

The National Day Rally is an annual address by the prime minister to the nation involving three separate speeches in three languages that takes about three hours in total.