SAN FRANCISCO — Google is expected to unveil the next generation of its Nexus tablets Wednesday in its latest challenge to Apple's market-leading iPad and Amazon.com Inc.'s Kindle Fire.

The new devices are likely to be the main attraction at a San Francisco event being hosted by Google Inc. executive Sundar Pichai, who oversees the company's Android and Chrome operating systems. The Nexus line of tablets that came out a year ago all run on the Android software that also powers hundreds of millions of smartphones.

Google hasn't revealed the reason for Wednesday's event, which is scheduled to start at 9 a.m. PDT.

But pictures of what's purported to be the upgraded versions of the Nexus tablet have been posted on the Internet during the past few days, fueling speculation that Google is poised to release the new devices.

The current versions of the Nexus tablets feature seven-inch and 10-inch displays and are set up to encourage users to buy music, video and applications from Google's Play store.

Since its introduction last summer, the seven-inch model has proven particular popular among consumers looking for a less expensive alternative to Apple's iPad. A Nexus 7 with 16 gigabytes of storage, sells for $199, the same price as a Kindle Fire HD with a seven-inch display. That compares to $499 for an iPad with a 10-inch screen and the same amount of storage. Google sell a Nexus with a 10-inch display and the same amount of storage for $399.

The popularity of the less expensive and smaller Nexus and Kindle Fire tablets is one of the main reasons that Apple released the iPad Mini with an eight-inch screen last year. Apple sells its smaller tablet for $329.