* Thousands start arriving in London for Olympic Games
* Security fiasco and transport worries cast shadow
* Busiest day on record at capital's main airport
By Peter Griffiths and Sophie Kirby
LONDON, July 16 (Reuters) - The first wave of Olympic athletes and visitors began pouring into Britain on Monday and officials played down fears that a packed London would buckle under the pressure of its biggest peacetime security and transport operation.
An embarrassing shortage of security guards, fears over airport queues and questions about the capital's creaking transport system have overshadowed preparations for the Games.
Extra soldiers were drafted in to help police the Games after private security firm G4S said it had run out of time to train all its newly recruited staff. The company's share price fell sharply on Monday.
Less than two weeks before the opening ceremony on July 27, Prime Minister David Cameron said the G4S shambles would not compromise Britain's largest peacetime policing exercise.
"We had contingency plans, we are using those contingency plans and we will do whatever it takes to deliver a safe and secure Games," Cameron told a news conference.
The security fiasco dominated the headlines over the weekend and raised fears that Britain woulod struggle to cope with the Games. A cartoon in the Daily Telegraph newspaper showed a red-faced, overweight runner wearing a G4S vest, complaining: "I didn't realise I had to run all the way to the end".
Some 12,500 police will be on Olympics duty each day, backed up by soldiers, fighter jets and missile batteries on the top of apartment blocks near the Olympics site in east London.
Security chiefs said they had prepared for threats on the scale of the Sept. 11, 2001 attacks. Four British Islamist suicide bombers killed 52 people on three trains and a bus in London on the day after the city was awarded the Games in July 2005.
London's main Heathrow Airport was ready for its busiest day on record, while the Olympic village opened its doors to the first athletes.
The first arrivals at an airport notorious for queues at security checks and passport control said everything had gone smoothly.
"I was expecting a three-hour queue like everyone said. It took not even five minutes. It was flawless - good job London," said John Retsios, 36, who had flown from New York with the U.S. Modern Pentathlon team.
Airport operator BAA Plc, owned by Spain's Ferrovial , said it expected a record 237,000 passengers to use Heathrow on Monday, including 335 athletes. The busiest day for arriving athletes is expected to be July 24.
The first section of road reserved for Olympic athletes and officials began operating on Monday, when one lane of the motorway linking Heathrow with the western edge of the capital was closed to all non-Olympic traffic.
It will form part of a 30 mile (48km) network of road lanes designed to whisk 82,000 athletes and officials through London's notoriously congested streets. Critics have nicknamed them "Zil lanes" after the roads reserved for the limousines of senior officials in the old Soviet Union.
London mayor Boris Johnson said the city was ready for the Games, the transport system would cope and visitors would be safe.
"When the opening ceremony begins, then a lot of these issues that we are currently discussing will melt away," Johnson told BBC television. "There is a bit of 'pre-curtain up' jitters and casting around for things to talk about."