SHANGHAI - Olympic champion Park Tae-hwan upset the parochial home crowd to lead from virtually start to finish to win the men's 400 metres freestyle gold medal at the world swimming championships on Sunday.

China's silver medallist Sun Yang, South Korea's gold medallist Park Tae-Hwan and Germany's bronze medallist Paul Biedermann (L-R) pose with their medals after the men's 400m freestyle final at the 14th FINA World Championships in Shanghai July 24, 2011. [Photo/Agencies]

The 21-year-old Park had only just scraped into the final, qualifying seventh, and was forced to swim in lane one from where he surprised the field to take an early lead that he only relinquished temporarily at the 200-metres mark.

"I am glad to swim in lane one, which allowed me to fully concentrate on my own tempo and not to get distracted from the competition," Park told reporters after clocking 3:42:04 on his way to victory.

Local favourite Sun Yang, who had recorded the fastest time of the year after he clocked 3:41.48 at the Chinese national championships in April, seemed content to sit behind the leaders and trust his kick, but was unable to reel in the Korean and finished second in 3:43.24.

"I have always considered him (Park) a strong competitor," Sun said. "The heats this morning showed that this is a high level competition (and it is) very good training for me ahead of the London Olympics."

Rome champion Paul Biedermann, who had broken Ian Thorpe's long standing 400 record in one of the now banned polyurethane suits in 2009, took bronze in 3:44.14.

It was a sweet redemption for Park, who had failed to qualify for the men's 400 metres freestyle final at the last world championships in Rome, despite being Olympic champion.

He had given South Korea its first Olympic swimming gold medal in Beijing and had been expected to take up the mantle of the then retired Thorpe as the dominant force in the men's 400.

After two years of hiccups, Sunday's victory suggested that he again could be the man to fill the Australian's shoes at the London Olympics next year.