UPDATE (July 30 at 9:00 a.m. ET): After two training sessions for the men's individual race were canceled due to high pollution levels in the River Seine (where the open swimming leg is to take place for all three contested triathlon events), the men's race has been rescheduled for Wednesday, July 31.

The women's individual race will commence July 31 at 2 a.m. ET. The men's individual race will follow, beginning at 4:45 a.m. ET.

This information is contingent upon fluctuating weather conditions and water levels, and subject to further developments. Keep checking NBCOlympics.com for the latest.

The men's triathlon at the Paris Olympics will not go ahead as planned on Tuesday as pollution levels in the Seine remain too high, World Triathlon said in a statement, dealing a blow to organizers and leaving athletes facing more uncertainty.

The race was postponed to Wednesday at 4:45 a.m. ET, immediately after the women's event, which is scheduled for 2 a.m. ET that day.

Organizers previously said they were confident water quality would improve in time for the race after heavy rains last Friday and Saturday dirtied the river.

"Despite the improvement of water quality levels over the last hours, the readings at some points of the swim course are still above the acceptable limits," they said early on Tuesday.

Should levels of bacteria remain too high by Wednesday morning both the men's and women's races are likely to be postponed to Friday, the contingency day reserved for the events.

If by Friday the water quality is still not good enough the swim leg will be scrapped and athletes will compete in a duathlon instead. For the mixed triathlon relay event on Aug. 5, the contingency day is Aug. 6.

Around 50 people had gathered on the sun-drenched Invalides bridge by 7 a.m. to watch the race, only to learn it had been postponed.

Magnus and Kristine Hagelsteen from Lund, Sweden, had packed pain au chocolats to have for breakfast while watching the race and cheering on Norway's Kristian Blummenfelt, who won gold at the Tokyo Olympics.

"It's a pity for everyone, but especially for the ones that are going to do the triathlon, I would be so disappointed - you're nearly there, ready to start, and then it's cancelled," said Magnus, who planned to come back on Wednesday.

Paris authorities have invested heavily to make the Seine swimmable as a key legacy of the Games, and spent 1.4 billion euros ($1.51 billion) on wastewater infrastructure to contain sewage and minimize spillage into the waterway.

Paris Mayor Anne Hidalgo took a dip in the river herself earlier this month, in a bid to convince doubters. City authorities have announced plans for three Seine swimming sites to open to the public by June next year.

"For the Parisians it will be fabulous if you can go swimming in the river, and it should be like this, it should be that you can swim in the river in the city," said Kristine Hagelsteen.

But the gamble that the river would be clean enough on the day of the triathlon was never guaranteed to pay off, especially as water quality varies widely day-to-day.

Bacteria aside, the Olympic triathlon course was conceived to maximize the wow factor, with the river swim a key element.

After diving into the Seine from a pontoon next to the Alexandre III bridge, athletes are set to race past the Musée d'Orsay and Grand Palais during the bike and run stages before ending back where they started.

Crossing the finish line on the bridge, they will be framed by stone columns topped with gilt-bronze statues of Pegasus, with the golden dome of the early 18th century Invalides monument as a backdrop.