American Katie Ledecky dominated the women's 1,500 metres freestyle event to claim her second gold medal at the swimming world championships in Budapest on Monday.

Ledecky, who won the women's 400 metres freestyle event on the opening day, cruised to the gold medal with a time of 15 minutes and 30.15 seconds.

The 25-year-old finished more than 14 seconds ahead of compatriot Katie Grimes who took the silver while Australia's Lani Pallister took the bronze.

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"I love this pool, I knew I put in the work to have a good swim and got into a rhythm," Ledecky said.

"I've got the relay, that's next on the agenda and then I will tackle the 800m. We'll see (if I can get a record)."

Earlier, Romanian teenager David Popovici smashed the world junior record as he powered to victory to take gold in the men's 200 metres freestyle event.

The 17-year-old became the first male Romanian swimmer to win gold at the world championships as he finished with a time of 1:43.21.

He finished more than a second ahead of South Korea's Hwang Sun-woo while Britain's reigning Olympic champion Tom Dean finished third.

"People at home believed in me. This medal is for them," said Popovici.

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"Now, I will go to recover and get ready for the 100-metre competition," he said looking ahead to the clash with Olympic champion Caeleb Dressel.

The women's 100 breaststroke race was without Olympic champion Lydia Jacoby and Tokyo silver medallist Tatjana Schoenmaker and Pilato took full advantage.

China’s Tang Qianting and Lithuania’s Ruta Meilutyte were fastest at the turn but it was tight going into the final stages with American Lilly King also in contention.

But Pilato touched first with a time of 1:05.93 to pip Germany's Anna Elendt by just 0.05 seconds while Lithuania's 2012 Olympic champion Meilutyte, returning from retirement, was third (0.09s behind).

"I'm super happy, it was my dream and it came true tonight," an emotional Pilato said as she broke down in tears, unable to finish her post-race interview.