Welsh set the record from lane eight More photos from Barcelona

Kitajima clocked 59.78secs in winning the men's 100m breaststroke with American Brendan Hansen finishing second in a time of 1min 0.37secs.

James Gibson finished third to claim Britain's first medal of the event.

Gibson was on record-breaking pace himself at the halfway point, but faded in the final 25m.

"It was hard," he said.

"I knew before I got in the water that it would be a real fight and I also predicted whoever won would have to take gold with a new world record.

"I fought for every inch of that pool and did as well as I could, but Kitajima is a great swimmer who had a stunning swim."

Kitajima's time of 59.78 in the men's 100m breaststroke eclipsed the old record of 59.94 set by Roman Sloudnov of Russia in Fukuoka in 2001.

Welsh won the 50m butterfly gold in a record time of 23.43secs, with American Ian Crocker second in 23.62 and Evgueni Korotychkine of Russia third in 23.73.

There was disappointment for Brtain's former bronze medallist Mark Foster, who finished seventh.

Welsh beat the old record of 23.44 set by fellow Australian Geoff Huegill two years ago at the last world championships in Fukuoka, Japan.

Kitajima's time eclipsed the old record of Roman Sloudnov

"I had no pressure in that race being a backstroker," said Welsh. "I was really happy with my start and the kick felt fantastic all the way through."

In the women's events, Jones set a new world mark of 1min 6.37secs in the 100m breaststroke semi-finals, beating the old record of 1min 6.52secs set in 1999 by Penelope Heyns of South Africa.

"I think it was seeing Matt set a world record that inspired me," said Jones.

In Monday's other medal events, American veteran Jenny Thompson won gold in the 100m butterfly and Yana Klochkova of the Ukraine won gold in the 200m individual medley.

Thompson, an eight-time Olympic champion, captured her first major individual gold since 1998.

Her time of 57.96secs beat silver medalist Otylia Jedrzejczak of Poland (58.22) and bronze medalist Martina Moravcova of Slovakia (58.24).

Klochkova won the women's 200 individual medley in 2min 10.75secs, the third fastest time ever.

Alice Mills of Australia claimed silver in 2min 12.75secs and Zhou Yafei of China took the bronze in 2min 12.92secs.