The device, which has a 9.7-inch color touchscreen for surfing the Web, watching movies, playing games and reading e-books, goes on sale in Germany, France, Italy, Switzerland, Spain, Britain and Canada later on Friday.
Apple has sold a million iPads in the United States since its April 3 debut, exceeding even the most bullish pre-launch estimates. Demand was so heavy the company had to delay the international roll-out by a month.
At Apple's flagship store in Tokyo's Ginza shopping district, about 1,200 people formed a line that stretched some 800 meters.
When a bell on the roof of the nearby Wako department store rang at 0800 local time, the doors of the Apple store opened to loud cheers from waiting customers.
I wanted to touch it as soon as possible. I felt real excitement when it was finally in my hands, said Takechiyo Yamanaka, 19, who camped out in front of the Ginza store from Wednesday evening to be the first in line.
Yamanaka was broadcasting himself on microblogging service Twitter while waiting and received encouraging words from Masayoshi Son, president of telecoms firm Softbank Corp, which sells the iPad in Japan, and others.
I felt quite relieved after I could finally get the iPad. I think I won't take my heavy laptop with me as often as I used to, said Shigeru Matsuyama, 48, a freelance writer.
The street was so packed with customers and media that police officials had to warn Apple employees to clear the sidewalk.
Enthusiasm about the iPad in Japan, the world's second-largest economy, is good news for Apple as international sales are increasingly important for the maker of the Macintosh computer and iPhone.
A model with 16 gigabytes of memory and Wi-Fi capability is being sold for 48,800 yen ($537) in Japan, compared with $499 in the United States.
On Wednesday, Apple shot past Microsoft Corp as the world's biggest technology company based on market value, the latest milestone in the resurgence of the maker of the iPhone, which nearly went out of business in the 1990s.
Apple now gets almost three-fifths of its revenue from overseas, and is seeing stunning growth in Europe and Asia.
Analysts said the iPad's sales in overseas markets were sure to match the success seen in the United States, helped by a large pre-existing base of Apple fans that already own the iPod or iPhone.
RBC Capital Markets estimates iPad's total shipments will reach 8.13 million units worldwide by the end of this year. Apple does not provide iPad sales forecasts.
But analysts also warned that Apple may struggle to supply enough of the devices and noted competition from a spate of competitors set to significantly expand the number of rival offerings in the tablet market this year.
It's a little bit hard to say because there's still going to be supply constraints, but I'm expecting them to sell every single thing they can ship, Andy Hargreaves, a U.S.-based analyst at Pacific Crest Securities, said ahead of the overseas launch.
On Tuesday, Dell unveiled its Streak tablet computer that can double as a mobile phone and will have a front-facing camera for videoconferencing. Sony Corp said on Thursday it would launch an e-reader in Japan by the year-end.
Apple also needs to attract normal customers on top of Apple lovers who automatically buy its products to keep the iPad selling.
I'm not going to buy the iPad now as it's expensive. And I'm a Sony fan, said college student Kengo Nakajima, 19.
The real game will start after 'core users' have the devices. I imagine a price cut may be necessary before the Christmas holiday season to stimulate demand, said Michito Kimura, a senior analyst at market research firm IDC Japan.
The iPad's global launch also comes as Apple struggles to ward off growing criticism about its secretive corporate culture, after it instigated an investigation into a lost or stolen prototype iPhone. That triggered a police raid on an Internet blogger's home and could result in felony charges.
Still, application providers and telecoms firms in Japan are keeping a close watch on the iPad's debut.
In Japan, where 476,000 iPads are expected to be shipped this year, according to RBC, Apple suspended taking pre-orders for the device on May 13 after only three days due to a supply shortage.
Dentsu Inc, Japan's biggest advertising firm, which operates an online book store to distribute magazines such as Newsweek Japan to smart phones including the iPhone, plans to distribute content to the iPad.
NTT Docomo, Japan's biggest mobile phone operator, will sell wireless LAN routers for the iPad's Wi-Fi connection next month to rival No.3 operator Softbank, which exclusively provides 3G networks for iPad.
(Additional reporting Alexei Oreskovic and Edwin Chan; Editing by Anshuman Daga)