American Katie Ledecky continued to pile up her gold medal collection on Monday after earning a record extending 17th world championship title with victory in the 1,500 meters freestyle.

Romanian teenager David Popovici confirmed his status as one of the most exciting emerging talents in the sport after producing a blistering swim to take gold in the men's 200 meters freestyle.

Italy enjoyed a memorable session at the Duna Arena with Thomas Ceccon breaking the men's 100 meters backstroke world record and 17-year-old Benedetta ­Pilato ­clinching the women's 100 meters breaststroke.

Ledecky, who won the women's 400 meters 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.

The win gave Ledecky her 20th world championship medal, equalling the women's record set by compatriot Natalie Coughlin.

"It's pretty wild because I feel like just yesterday I was in Barcelona at my first Worlds. It's awesome representing Team USA year after year, and it's an honor and a privilege, and it's why I work hard," she said.

Popovici, 17, has been drawing comparisons with some of swimming's true greats with his dominant performances in junior championships and outpowered a strong field in the 200 meters free for a victory which announced his arrival in the elite.

The Bucharest native, who finished fourth in Tokyo, became the first male Romanian swimmer to win gold at the world championships as he finished with a time of 1:43.21 - a new junior world record.

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.

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

Italy's joy began when Ceccon, 21, finished the 100 meters backstroke with a phenomenal time of 51.60 seconds - 0.25 seconds better than American Ryan Murphy's previous mark.

Murphy, a four-time Olympic gold medalist, finished second while fellow American Hunter Armstrong was third.

"I have no words, there was an Olympic champion in 2016 [Murphy] and other strong people [in the field]. I'm pretty happy," said a speechless Ceccon.

The women's 100 meters 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.