Opera Mini for the iPhone was downloaded more than one million times on its first full day of worldwide availability.

Opera Software said it was the number one free application in the 22 international iTunes App Stores featured on Apple.com.

The mobile Safari alternative started landing in iTunes App Stores as early as Monday, and was available worldwide by Tuesday afternoon.

Early impressions of the mobile browser were mostly good, with reviewers saying it was significantly faster than Safari on the iPhone.

Critics also said Opera had some great features that Safari lacked, such as the ability to search for keywords in a web page and the ability to adjust image quality settings.

Many reviewers did note, however, that Opera Mini lacks some of the functionality of Safari, such as refined pinch-to-zoom controls and the ability to add web links to the iPhone's home screen.

Will these numbers last?

"Today iPhone users have a choice, and, as the numbers show, they are eager to explore new and faster ways to surf the Web on the iPhone - especially during heavy web traffic," Opera Software CEO Lars Boilesen said on Thursday in a statement.

But how much of a choice do iPhone users really have? A key component of any browser is the ability for a user to designate it as the default browser for basic things like opening links to web pages.

But on the iPhone a user's only choice for default functionality is Safari.

As it stands right now, there is no way to change the default browser on the iPhone from Safari to Opera or any other browsers that might come to the iPhone at a later date.

By the numbers, Opera may be a popular iPhone download right now, but a more interesting metric would be to take the number of web page requests coming from Opera for iPhone in the next week, and then compare that to the number of Opera for iPhone page requests over seven days in mid-May.

Users may be excited to try the new mobile browser right now, but my guess is interest in Opera for iPhone will be hard to sustain for a browser that cannot be integrated into its operating system.

See also: Opera Mini for iPhone submitted to App Store