By Alan Baldwin

RIO DE JANEIRO (Reuters) - Britain's Adam Peaty smashed his own 100 meters breaststroke world record and Hungary's 'Iron Lady' Katinka Hosszu had another in her sights on an explosive opening day in the pool at the Rio Olympics.

American golden girl Katie Ledecky meanwhile set herself up for the first medal of a likely hefty haul after swimming the final leg of the 4x100 freestyle relay.

On a normal day Ledecky might have been the center of attention, but Peaty seized the limelight like a man on a mission.

The 21-year-old released plenty of pent-up energy to ensure a sensational start on his Olympic debut, completing the first 50 meters 0.35 seconds inside world record pace.

The crowd raised the roof as he touched out in 57.55.

The world champion had set the previous record of 57.92 in London in April 2015 and is favorite to become Britain's first male swimming gold medalist since Adrian Moorhouse in the same event in 1988.

Peaty, who was not even born then, has another swim in the evening semi-finals before Sunday's final.

"Hopefully tonight I'm going to feel a little bit better and swim a little bit faster... We're going to keep pushing the boundary," he told reporters.

Swimming earlier, Hosszu might have beaten Peaty to the honor of setting the first record in Rio but eased off in the women's 400 meter individual medley just as it was in her sights.

The 27-year-old, who was well inside the four minutes 28.43 seconds set by China's Ye Shiwen at the 2012 Games until the last 50 meters, still set the second fastest time ever with a European record 4:28.58.

Ye failed to qualify, ending up 27th.

Hosszu -- world champion in 400 IM, a two times female swimmer of the year and winner of five world championship golds -- is still seeking her first Olympic medal in her fourth Games but looks a solid bet this time.

In the men's 400m IM heats, Japan's world champion Daiya Seto and compatriot Kosuke Hagino had to settle for second and third with American Chase Kalisz setting the pace in what promises to be a close battle for gold.

South Korea's Park Tae-hwan, the 2008 champion who won a late battle to swim at the Games after appealing against a doping ban, failed to qualify for the men's 400 meters freestyle finals.

The 2012 silver medalist was 10th overall with only eight going through. China's world and Olympic champion Sun Yang was fourth fastest overall in a line-up led by Conor Dwyer of the United States.

(Reporting by Alan Baldwin, editing by Toby Davis)