(Adds final medal, reports of skating controversy)
* First full day of sporting action in Sochi
* American Sage Kotsenburg claims first gold of Games
* Judging controversy dismissed by Americans
By Mike Collett-White
SOCHI, Russia, Feb 8 (Reuters) - Russia's Winter Olympics sprang into action on Saturday with the first five gold medals decided, and the world began to forget the security fears and political wrangling that had marred the buildup to the most expensive Games ever staged.
The United States claimed the first gold in the slopestyle event held amidst the snow-capped peaks of the Caucasus Mountains, some 40 km (25 miles) northeast of the Black Sea resort town of Sochi where the main Olympic park is located.
On a clear, crisp day, U.S. snowboarder Sage Kotsenburg leapt and twisted high in the air, riding an almost flawless first run to win the slopestyle crown.
Rock music blared and fans danced as the event, which bears similarities to skateboarding, made its debut in an initiative to attract a younger demographic to Winter Olympic sport.
"I have no idea what's actually going on. This is the craziest thing that ever happened," Kotsenburg said.
The only hint of a blemish on an enthralling day of sport were reports suggesting the United States and Russia were conspiring to help each another win figure skating gold medals, even though such an arrangement would seem to make little sense.
"Comments made in a L'Equipe story are categorically false," the American governing body, U.S. Figure Skating, said in a statement on Saturday. "There is no 'help' between countries."
Several U.S. media picked up on a report saying the arrangement would help Meryl Davis and Charlie White to become the first Americans to win the Olympic ice dance gold, while Russia in return would benefit in team and pairs competitions.
Judging scandals have often blighted figure skating, the most famous being at the 2002 Salt Lake City Olympics when allegations of vote-rigging led to Canadians being awarded duplicate golds in the pairs competition with Russians.
Russian President Vladimir Putin has staked his reputation on hosting a successful Games, preparations for which have been overshadowed by threats from Islamist militants based in nearby Chechnya and other north Caucasus regions to disrupt them.
The leader has also been under fire for legislation passed last year banning the promotion of gay propaganda among minors, which critics say curtails the rights of homosexuals in Russia.
Putin says it is designed to protect young people, and has stressed homosexuals would not face discrimination in Sochi.
Some 37,000 security personnel are on high alert over militant threats to attack the Feb. 7-23 Games, the most expensive ever held at an estimated cost of $50 billion.
Security analysts believe an attack is more likely elsewhere in Russia to humiliate Putin, who launched a war to crush a Chechen separatist rebellion in 1999.
Twin suicide bombings killed at least 34 people in December in Volgograd, 400 miles (700 km) northeast of Sochi.
Now the Games are up and running, Putin hopes they will project Russia as a resurgent power that has finally shed the shackles of its Soviet past.
The opening ceremony on Friday night, before 40,000 spectators and millions more watching on television, tried to do just that, taking viewers on a race through Russian history with spectacular set pieces on a gargantuan scale.
The event, at the gleaming Fisht Stadium newly built on the shores of the Black Sea, was not without its problems, however.
One of the five Olympic rings failed to open properly, and when Russian state television noticed, footage of a successful rehearsal was used instead to conceal the glitch.
On Saturday, Russia's Games chief Dmitry Chernyshenko was questioned about the choice of triple Olympic figure skating champion Irina Rodnina to light the flame - a huge honour in the sporting world.
A national hero in Russia and member of parliament loyal to Putin, she had caused an outcry in the United States last September by re-tweeting a photoshopped picture showing President Barack Obama chewing and a hand waving a banana in front of him.
"Irina Rodnina is from the club of three-times gold medallists and one of the most respected in the world," Chernyshenko told reporters. "The Olympic Games are not about politics," he said.
There were plenty of empty seats at Sochi venues on Saturday. Organisers said about 70 percent of tickets had been sold for the Games overall, and that some venues' seating capacity had to be reduced for security reasons.
Those who did show up were treated to some memorable moments, notably when Canada's Justine Dufour-Lapointe won the women's moguls freestyle skiing gold medal ahead of her sister Chloe, who took silver.
Norwegian Marit Bjoergen, dubbed the "Iron Lady" and the most successful athlete of the 2010 Games with five medals, won the cross-country skiathlon.
Sven Kramer of the Netherlands successfully defended his 5,000 metres title in an Olympic record time at the Adler Arena.
And Norway's Ole Einar Bjoerndalen matched compatriot Bjorn Daehlie's Winter Games record of 12 medals when he won the biathlon men's 10 km sprint. At 40, he became the oldest Winter Games gold medallist.
Looking ahead to Sunday's big draw, the men's downhill, American Bode Miller laid bare the dangers faced by competitors when he said the treacherous Rosa Khutor piste "could kill you". (Additional reporting by Karolos Grohmann, Timothy Heritage and Keith Weir in Sochi, Martyn Herman, Julien Pretot, Nick Mulvenney and David Ljunggren in Rosa Khutor and Alexei Anishchuk in Bocharov Ruchei; Editing by Ossian Shine)