Fans stormed Apple Inc's flagship store in Japan and operator Softbank Corp's outlets as the company launched a slim new version of its wildly popular iPhone.

Due to the time difference in the global launch, Japanese stores were the first to get hold of the coveted iPhone 4, which debuts in the United States, France, Germany and the UK later on Thursday.

Sales of the iPhone 4, which boasts a higher-quality screen and longer battery life than the previous model, blew past expectations in its first day of pre-orders last week with more than 600,000 sold in just 24 hours.

I can't stop smiling, said Noboru Takahashi, a musician who had waited in line since Monday at Softbank's Omotesando store in a posh shopping district of Tokyo. He was the first in a queue of more than 300 people.

Softbank is the exclusive iPhone operator in Japan.

The iPhone has been a huge success since it debuted in 2007, boosting Apple's margins and transforming it into one of the world's leading mobile device makers.

The iPhone, however, faces a slew of new competitors, including Motorola Inc and HTC, designing high-powered handsets based on Google Inc's Android software.

The launch of the iPhone 4, which also offers video chat via Wi-Fi, and a gyroscope sensor for improved gaming, comes less than a month after Apple's iPad made its splashy debut in international markets.

For the current quarter, which ends June 26, analysts roughly expect Apple to sell 8-9 million iPhones in total, which includes sales of older generation models. IPhone 4 sales in the current quarter are expected to be limited by supply shortages.

Going forward, analysts expect the company to ship 10 million or more a quarter, as production ramps to meet demand.

(Reporting by Sachi Izumi; Editing by Anshuman Daga)