Trans swimmer Lia Thomas booed after winning race but runner-up receives loud cheers.

Lia Thomas has become the first transgender athlete to win a National Collegiate Athletic Association swimming championship - with her controversial victory attracting a mixed reception.

The UPenn swimmer, 22, won the 500 yard freestyle in Atlanta in a time of 4 minutes, 33.24 seconds on Thursday evening.

Afterwards, the crowd were notably more enthusiastic when cheering for the woman who'd come second place - Emma Weyant, of the University of Virginia. She swam 4:34.99.

While Thomas was given some cheers, boos could also be heard ringing out throughout the spectator stands, as she continues to face allegations that going through male puberty has given her an unfair advantage over her rivals.

'I try to ignore it as much as I can, I try to focus on my swimming what I need to do to get ready for my races and I just try to block out everything else,' said Thomas after the race, when asked by ESPN about the response.

'It means the world to be here, to be with two of my best friend and teammates and be able to compete.'

Thomas, whose continued wins and record-breaking performances have made her the world's most controversial athlete, also roundly defeated fellow swimmers at last month's Ivy League championships.

The Texan, who swam for three years on the university's men's team before transitioning in 2019, is now the first transgender athlete to win a NCAA championship - a distinction one of Thomas' teammates said would be dubious if achieved.

Thomas has undergone the required hormone treatment to meet the current rules for transgender athletes, but critics say her stunning performances prove that she still retains a considerable and unfair advantage.

'It's not necessarily an achievement in my mind,' said one of Thomas' teammates on UPenn's Women's Swim Team.

The teammate, who refused to give her name for fear of repercussions, told Fox News Digital that Thomas's participation in D1-sanctioned women's events has 'completely ruined the integrity of the sport.'

She said Thomas' achievements while on the women's team should be taken with a grain of salt, due to the biological advantages of being born a man.

'It's its own distinct category because no woman is going to be as fast as a man, and here, is just completely - we're just throwing away the definition of a record to fit into someone else's agenda of what it should mean to them,' she said.

'In reality, it makes no scientific sense to do so.'

Thomas is one of more than 300 swimmers who qualified for the NCAA championships this week, after securing a trio of records at the Ivy League Championships last month in the 100, 200, and 500 yard freestyle events.

Thomas is set to compete in those same events at this week's NCAA championships.

In two of them, she ranks first in the country.

'It's still just disappointing to know that the NCAA lacks the courage to do the right thing,' said Thomas' teammate.

The NCAA leadership decided in January to amend its policies concerning trans athletes, to allow each sport's governing body to discern whether an athlete is eligible to compete.

Under the new guidance, Thomas was allowed by USA Swimming to compete as a woman because she has completed a year of hormone treatment. Shortly thereafter, USA Swimming announced a new requirement that transgender women must suppress their testosterone levels for three years before competing - a rule which would have seen Thomas excluded from future female competitions.

It appeared at the time that Thomas would be barred from the NCAA championships as a result, after the NCAA's assertion that they would abide by USA Swimming rules.

But just before last month's Ivy League competition, the NCAA backtracked on its decision, saying that instituting a new policy in the middle of the season would be unfair - allowing Thomas to compete at that competition as well as this month's championships.

Her continued participation in women's competition has proved deeply divisive, with former Olympian Caitlyn Jenner - who won gold in the decathlon as Bruce Jenner - among those criticizing Thomas for swimming in women's races.