Apple on Wednesday said it had more than 600,000 iPhone 4 pre-orders yesterday as customers shutdown AT&T’s ordering systems.

Apple said it was “the largest number of pre-orders Apple has ever taken in a single day and was far higher than we anticipated, resulting in many order and approval system malfunctions.”

Apple also acknowledged the problems many customers had trying to pre-order the iPhone. “We apologize to everyone who encountered difficulties, and hope that they will try again or visit an Apple or carrier store once the iPhone 4 is in stock.”