People have to be prepared for some difficulties when one of the busiest cities in the world is hosting the Olympics, the Prime Minister said.
"This is an extraordinary few weeks for London," Mr Cameron said.
"We can't say to people that life is going to be completely as normal, it isn't.
"This is an extraordinary few weeks for our city, for our country, and I think people have to be prepared for some difficulties as a result of that."
He went on: "We are holding an Olympic Games in one of the busiest, most active, bustling cities anywhere in the world.
"Of course it's easier if you hold an Olympic Games in the middle of nowhere.
"Inevitably you're going to have challenges".
There were "immense challenges" at Heathrow Airport, he added, but the reports were that people were coming through there "very, very quickly".
"Obviously there are transport challenges in London, but I think the flexibility that has been shown about the Games Lanes, only restricting them to the Games traffic when it's necessary - of course that makes it more complicated, but I think that's showing real flexibility and is right."
Mr Cameron added that he travelled to the Olympic Park on the Jubilee line this morning.
Referring to the public transport system, Mr Cameron added: "Of course it's going to be challenging, but so far I would argue it's held up well."
He added: "There are always lessons to learn from your country's preparations.
"Clearly, the difficult issue of the last two weeks of getting the security right - we've had to act and act quickly to make sure that everything is working properly."