The naked and muddy kick off Danish music festival

ROSKILDE, Denmark (Reuters Life!) - The largest music festival in the Nordics kicked off on Thursday with fans braving knee-deep mud to see bands such as classic rock giants The Who and California funk-rockers Red Hot Chili Peppers.

Roskilde festival organizers said 75,000 tickets had been sold, with 45,000 fans arriving as early as Sunday at the grounds half an hour west of Copenhagen to find warm-up acts playing in front of a massive mud bath.

Last month was the wettest June in recorded history in Denmark, and fans are not likely to get a respite from the rain, with plenty more expected on Thursday, meteorologists said.

"This rainy weather kills the ambience, because people are staying in their tents instead of partying," said festival-goer Christiane Sorensen, at Roskilde for her fourth time. "But I will defy the horrific weather and see as many bands as possible."

Organizers pleaded with fans not to drive to the festival, as all the parking areas were flooded and rubber boots were sold out in all stores in the Copenhagen area.

Many fans dispensed with rain gear and embraced the mud. Ad-hoc wrestling competitions proved a popular early activity, with some contestants taking part completely naked.

The festival, founded in 1971, features more than 170 bands on seven stages, including U.S. rap pioneers Beastie Boys, eclectic Icelandic singer Bjork, Las Vegas rockers The Killers, alternative British groups the Arctic Monkeys and Muse, Montreal buzz band Arcade Fire, New York dance-rockers LCD Soundsystem and California rockers Queens of the Stone Age.

Other attractions include films, spoken word performances and skateboarding. The rain and mud might deter fans from beach volleyball, but could prove a boon for planned kayak polo games.

In a nod to the Danish tradition of tolerance for youthful rebellion, the festival offers a graffiti workshop and a makeshift Youth House for a group of leftwing young squatters whose March eviction from a Copenhagen building sparked riots.

Four fans were injured on Tuesday when a camping gas canister exploded, providing a dreadful flashback to the 2000 festival when eight fans were trampled to death at a crowded performance by grunge veterans Pearl Jam.

Much to the organizers' relief, this year's injured were released from the hospital after only one day.

(Additional reporting by Martin Burlund)