London (CNN) -- The Dutch women's beach volleyball team sailed through Heathrow airport as it arrived in London for the Olympic Games, but not every athlete had such a smooth arrival.
American hurdler Kerron Clement's bus got lost on the way from the airport to the Olympic village, he said, resulting in a four-hour-plus journey.
"Athletes are sleepy, hungry and need to pee. Could we get to the Olympic Village please," the world record holder said on Twitter on Monday. "Not a good first impression London."
The drive should take about an hour.
London's Olympic organizers, London Organising Committee of the Olympic and Paralympic Games, said "the vast majority" of bus journeys had gone smoothly, but conceded that "there may have been one or two journeys taking longer than expected."
Heathrow airport is expecting a record number of passengers on Monday as athletes begin to flood into the city for the Olympic Games, which start a week from Friday.
The arrivals begin amid a security snafu, as the government prepares to call in 3,500 military personnel because the security contractor says it does not expect to have enough staff in place.
G4S, the contractor, admitted last week that it would not be able to provide more than 10,000 security staffers by the time the Olympics begin.
The staff were supposed to be doing tasks including venue perimeter security such as manning X-ray machines, searching people, searching vehicles, and operating closed-circuit television systems, G4S told CNN on Sunday.
Home Secretary Theresa May, who is responsible for domestic security, was called to Parliament to answer questions from lawmakers on Monday after the fiasco.
She insisted that G4S actually had more than 20,000 accredited security staff, and that until last week it appeared they would have too many contractors rather than too few.
The Home Office told CNN earlier on Monday that the contractor was suffering from a software problem which meant they could not guarantee who would turn up where, and whether guards had the right training.
G4S did not respond immediately to a CNN request for comment on the accusation.
The Home Office also said that the extra immigration staff deployed at borders was properly trained, rejecting media reports to the contrary.
Immigration desks will have extra staff, Heathrow said Monday, amid fears of long lines to get into the country.
Retired border officials and retired police officers are among those being brought in to help immigration staff, the Home Office said.
The airport expects nearly 237,000 people to fly in or out on Monday, about 25% more than on a normal day.
The arrivals include 335 athletes, the airport said in a statement. Athlete arrivals are expected to peak on July 24, with more than 1,200 competitors due on that day.
Cubans, Italians and Russians were among the arrivals on Monday.
Pressed about the security snafu, Sports Secretary Jeremy Hunt said Sunday it should be no surprise that some contractors were unable to meet their commitments.
"It's completely normal that you're going to find some contractors on a project of this size who aren't going to be able to deliver what they promised," he said on the BBC's "The Andrew Marr Show."
He praised the contractor as being "honorable" for having admitted the problem, apologizing and covering the costs of bringing in military personnel.
And he reiterated that the government learned only last week that G4S would not meet its commitment.
"Management told us right up until last week that everything was on track," Hunt said.
The contractor said Saturday it stands to lose up to $77 million after failing to recruit enough staff.
The airport, meanwhile, said Monday it is deploying more than 500 volunteers who speak 20 languages among them to welcome athletes and officials.
The Games start on July 27.
CNN's Jo Shelley, Jim Boulden, Dan Rivers, Stephanie Halasz and Erin McLaughlin contributed to this report.