The first version of the iPhone was snapped up by 270,000 people within days of its June 2007 U.S. launch.
Analysts expect the new iPhone to draw as many as 10.5 million buyers worldwide this year and with 6 million of the older devices already sold, help Apple beat its target of selling 10 million devices by the end of 2008.
In the United States, many consumers were excited by the iPhone's promise of faster Web speeds and applications such as a friend tracker service. But some complained Apple's online store, where consumers can download games and other software for their phones, was not working on Friday morning, apparently overwhelmed by demand.
"I really did want to see what the new phone was about. I'm hoping there is a big improvement from the old phone," said Winston Yang, 37, a tech project manager outside the Apple store on Fifth Avenue in New York.
While most carriers are shouldering some of the cost of the phone for customers who sign long-running contracts - some are opening the phone to consumers who want to pay for calls in advance and charging those customers more for the device.
"Opening it up to prepay outside North America is very important . prepaid is the tariff of choice for most people in most parts of the world," CCS Insight analyst Shaun Collins said.
Outside one Brussels store, where the iPhone is sold unlocked and without a contract beginning at 525 euros (419 pounds), a queue, all men, was steadily building an hour before opening time on Friday.
Germain Merinero, a European Commission employee, at the head of the queue said he was not put off by the price.
"It's a little toy and you pay what you have to," he said.
But a midnight launch in Helsinki, the home turf of the world's biggest mobile phone maker Nokia, drew a scant crowd, while outdoor bars nearby were booming with business.
TASTE OF THE ACTION
The next-generation iPhone is Asia's first official taste of the touch-screen device previously available only in the United States and Europe.
Many analysts doubt the device will be popular among mainstream customers in Japan, Asia's largest retail market, because it does not support television services or electronic payment features widely used in the country.
Others point to a vibrant grey market for fakes or unlocked phones - hacked to work on the networks of other carriers - in China and southeast Asia, cannibalising demand.
Outside of London's Apple store, not everyone was infected by the hype. Looking at the hubbub surrounding the store, one passerby commented: "What's the big deal?"
