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The 500px iPhone app had almost a million downloads when it was pulled from the app store. (500px)

Photography apps from a Toronto startup have been pulled from Apple's app store over concerns that they "featured pornographic images" and "complaints about possible child pornography," Apple says.

500px Inc.'s  "500px for iOS,"  which allows people to share and showcase images, was one of two apps from the Toronto-based developer pulled from the app store on Tuesday, to the dismay of the company's chief operational officer, Evgeny Tchebotarev.

"After almost 1M downloads, @500px was removed from Apple's App Store. Why?:(" Tchebotarev tweeted early Tuesday morning, when the first of the two apps was removed.

500px is one of six nominees for best overall Canadian Startup of 2012 in the 2012 Canadian Startup Awards, with the winner to be announced on January 28.

In statement provided CBC News, Apple explained: "The app was removed from the App store for featuring pornographic images and material, a clear violation of our guidelines. We also received customer complaints about possible child pornography. 

"We’ve asked the developer to put safeguards in place to prevent pornographic images and material in their app."

For the developer's part, the app only allows artistic nudity, and users, by default, do not have access to even those nude images unless they log in through a desktop computer to turn off a "safe search" setting, according to Tchebotarev in numerous statements to different media outlets

By mid-Tuesday afternoon, the company had submitted an updated app for review, Tchebotarev tweeted.

However, 500px's "ISO500" photo app, produced by Pulpfingers, a small startup acquired by 500px in November, was temporarily removed from the App Store later on Tuesday, according to a tweet from the @pulpfingers account.

Pulpfingers indicated in a tweet on Wednesday that it had already submitted an update to Apple's app story that "fixes a few bugs and deals with the nudity concerns."