The leader, known as Ja New, or Sirawith Seritiwat, was about to take the stage to speak with the demonstrators when he collapsed.
He regained consciousness following treatment, and blamed the weather, telling the crowds: "the weather is so hot, so I fainted. I'm okay now."
He then went up to the stage and addressed the cheering demonstrators.
The protesters, led by the "We Want Election" group, left the university compound through the Sanam Luang gate and another leader, Rangsiman Rome, later went to the front of the line of marchers, asking to talk to the officer in charge. He however failed to get the green light to move towards Government House.
Rangsiman said the protesters would stay put on the road until they were allowed to proceed to Government House.