LONDON — As Olympic organizers saw it, Monday was to have been the day when things finally started to go right in a countdown to the Games that has been something of a public relations disaster, mostly given the fiasco over too few security guards — thousands too few — and the emergency deployment of British troops to fill in.

And in many ways, matters went swimmingly — until, as the first contingents of athletes were transported across London to the Olympic Village, they went spectacularly wrong.

By the end of the day, organizers were struggling to explain how three buses carrying dozens of athletes, officials and journalists to the Olympic Village from Heathrow Airport lost their way in the maze of London's streets, causing one U.S. medal hopeful, 400-meter hurdler Kerron Clement, to post a Twitter message in desperation after four hours aboard a bus that should have made the 24-mile journey in 45 minutes.

"Athletes are sleepy, hungry and need to pee. Could we get to the Olympic Village please," he wrote as the driver, struggling to understand the route given by the bus' GPS device, finally abandoned repeated forays up dead-end streets and pulled out a map.

"Not a good first impression London," Clement added.

The misrouted buses episode came as thousands of competitors and team officials began arriving at Heathrow on a day that officials said was the busiest in the airport's 60-year history. The airport handled about 236,955 arrivals and departures Monday, breaking the record of 233,562, set on July 31, 2011. Another big arrival day will be July 25, two days before the opening ceremony.

Officials noted that while a few buses might have gotten lost temporarily, hundreds of others managed to get from Heathrow to east London just fine.