Tamara Potocka, a swimmer from Team Slovakia, needed medical assistance after competing in the women's 200-metre individual medley.
The 21-year-old athlete collapsed at the side of the pool after finishing seventh in the race, according to Sky News.
Medical teams rushed to assist her and images show that she was seen with an oxygen mask as she was taken away on a stretcher.
The manager of the venue has confirmed that Potocka is conscious and is currently undergoing medical assessment.
It remains unclear whether the athlete had to receive CPR.
Eight swimmers had already taken to the blocks for the next heat when Potocka required medical attention, prompting race organizers to halt the event and send the swimmers back to their seats upon realizing the severity of the situation, per the Sun.
Commentators watched in horror as the incident unfolded.
One person remarked: "This doesn't look good... 15,000 people are now watching an athlete surrounded by medical staff and I can assure you it's absolutely silent in here."
"They've got a stretcher out, they've got an oxygen mask on the athlete, and I'm going to say... this looks very serious."
"They've got the neck brace around so I can only imagine that someone has hit their head and has some kind of trauma - they want to immobilize the athlete. We hope that there's nothing more than that..."
She finished the heat with a time of two minutes 14.20 seconds which was not enough to secure her place in the next round.
Canada's Summer McIntosh won heat three with a time of 2:09:90 and was followed by Yiting Yu of China, Australia's Ella Ramsay, and Team GB star Abbie Wood.
The fastest 16 swimmers from the heats progress to the semi-finals which are due to take place on Friday evening.
This is a breaking news story, more to follow...
