(Updates with swimming wins, adds details)
* Swimming surprises cap dramatic day at Games
* French, Lithuanian victories against the odds
* Questions over Chinese swimmer's stellar times
* Swiss soccer player expelled for racist tweet
LONDON, July 30 (Reuters) - Lithuanian swimmer Ruta Meilutyte struck Olympic gold on Monday in the women's 100 metres breaststroke aged just 15, capping a dramatic day in the pool where Frenchman Yannick Agnel also beat an American favourite to win the men's 200 metre freestyle.
On the third full day of competition in London, Swiss soccer player Michel Morganella was expelled for an abusive message on Twitter after his team's defeat by South Korea.
And Chinese swimming sensation Ye Shiwen's world record win in the 400 individual medley on Saturday raised eyebrows in the world's media about the 16-year-old, prompting her to deny that she had taken performance-enahncing drugs.
But it was heroics in the water that set ablaze the Olympic village, and in particular those of Meilutyte, the first swimmer from her country to win an Olympic medal.
The Lithuanian had to do it the hard way, surviving a fierce challenge from American Rebecca Soni, the reigning world champion in the event, and just holding her off at the death.
"I can't believe it," a stunned Meilutyte managed to say in a post-race poolside interview.
Agnel, a 20-year-old who reads the poetry of Charles Baudelaire between races to relax, captured a second gold medal in 24 hours after his stunning anchor leg won France the 4x100 freestyle relay the previous night.
And he had to beat a stellar field including U.S. world champion Ryan Lochte who ended up fourth.
The United States caught up some ground on China in the medals rankings, however, with two late golds in the pool.
Matt Grevers collected his first individual gold in the final of the men's 100 metres backstroke while teenager Missy Franklin won the women's 100 backstroke.
It took the U.S. gold tally to five by the end of the third full day of competition, still four golds behind medal table leaders China on nine.
American Michael Phelps has a chance to make history on Tuesday by becoming the most decorated Olympian if he scores medals in two men's finals.
It would take his tally to 19, one more than the current all-time record of 18 held by Soviet gymnast Larisa Latynina.
At a press briefing at the main Olympic site in London's East End, reporters asked Arne Ljungqvist, International Olympic Committee medical chief, whether Chinese Ye's sensational victory was in any way suspicious.
"I say no," replied Ljungqvist, who has 40 years experience in anti-doping. "Should a sudden raise in performance or a win be primarily suspect of being a cheat then sport is in danger because this ruins the charm of sport," he added.
Ye, who turned 16 in March, powered to gold in the 400 metres individual medley at the weekend and became the first female swimmer to break a world record since the ban of high-tech suits, taking over a second off the previous benchmark.
She brushed aside doping suspicions, saying Chinese athletes were clean.
"My results come from hard work and training and I would never used any banned drugs. The Chinese people have clean hands," she told reporters.
At the first Games where social media has become such a major issue, Switzerland's Morganella was expelled for an offensive tweet.
It follows the withdrawal last week, before the July 27-Aug. 12 tournament got under way, of Greek triple jumper Paraskevi Papachristou for another tweet deemed racist.
A row over empty seats at venues across London rumbled on, with Olympic organisers under pressure to fill arenas and placate a public furious at seeing TV pictures of unused places, having been told months ago that venues had sold out.
British Prime Minister David Cameron said volunteers, soldiers and the public would be able to take some of the available places, but added: "You'll never have complete eradication of empty seats."
Ticketing confusion also led to the opposite problem - overcrowding - in at least one instance on Monday.
Dozens of angry ticketholders trying to get into the men's 10m air rifle competition at Royal Artillery Barracks were turned away because the venue was too full.
But London's transport system largely defied predictions of gridlock on the first regular working day of the 2012 Games.
Transport bosses expect an extra three million journeys per day on top of the usual 12 million during the Games, an Olympian test for an underground train network that first opened in 1863 during the reign of Queen Victoria.
On the first morning rush hour since the Games opened on Friday night, commuters said buses, trains and the metro were working surprisingly smoothly with a few hiccups, and roads were generally clear.
Chris Round, 23, from Boston, Massachusetts, took the Underground and Docklands Light Railway to watch the judo.
"It was real easy to get to," he said. We just got on the first train that came. It was kinda crowded but it wasn't bad."
Host nation Britain was still waiting for its first gold medal, but a bronze in the men's team gymnastics felt almost as good as it ended a 100-year wait for any medal in the event.
Princes William and Harry, grandsons of Queen Elizabeth, led the support at a raucous North Greenwich Arena, and the loud cheers were muted only slightly when an initial silver medal was downgraded following an appeal by the Japanese team. Other royals were also at the event.
In other medal action, China's Cao Yuan and Zhang Yanquan won the men's synchronised 10 metre platform diving and compatriot Li Xueying broke two Olympic records on the way to the women's 58 kilogram weight division in the weighlifting.
Romania took its first gold with Alin George Moldoveanu's surprise victory in the 10m air rifle event, and on a packed day of sport Russia's Mansur Isaev triumphed in the 73kg men's judo after a fast and furious fight with Japan's Riki Nakaya. (Additional reporting by Karolos Grohmann, Michael Holden, editing by Peter Millership)