SAN FRANCISCO » Google is countering the release of Apple’s latest iPhones with two devices running on a new version of Android software designed to steer and document even more of its users’ lives.

The Nexus 6P and Nexus 5X unveiled Tuesday are the first smartphones sold with an Android upgrade called "Marshmallow." It features changes that will give expanded powers to Android’s personal assistant, Google Now, so it can explore the information that people call up in the mobile applications.

Previously, Google Now learned about its users’ interests and daily habits by analyzing search requests and scanning information contained in emails.

The Marshmallow version of Android enables users to summon Google Now to scan whatever content might be on a mobile device’s screen so it can present pertinent information about the topic of a text, a song, a video clip or an article.

The new Android feature, called "Now on Tap," will be activated by holding down the device’s home button or speaking, "OK Google," into the microphone. That action will prompt Now on Tap to scan the screen in attempt to figure out how to be the most helpful. Or, if speaking, users can just say what they are seeking, such as "Who sings this?"

Google began taking pre-orders for the Nexus phones in the U.S., United Kingdom, Ireland and Japan on Tuesday with prices for the 6P starting at $499 and $379 for the 5X with no commitment to a wireless contract required. They are expected to start shipping by mid-October.

Google uses its Nexus line of phones to showcase how the company would like Android to be set up. Most phone makers, though, still take advantage of the freedom that Google gives them to alter Android, although the free software usually is still set up to drive traffic to Google’s search engine, YouTube video site and other digital services.

Marshmallow also features Google’s revised mobile payment system, Android Pay, and more privacy controls, including support for fingerprint scanning for phones such as the new Nexus models equipped with that technology.

"In a multiscreen world, it is even more important for Google to do the hard work so the experience is simple and delightful for the users," Google CEO Sundar Pichai said.

The Nexus phones also feature better cameras than previous models, matching similar upgrades that Apple just made to its iPhone 6s and iPhone 6s Plus.

Google Inc. is promising the Nexus 6P will be able to be fully charged in half the time that it takes the iPhone 6s Plus.

The Nexus 6P has a 5.7-inch screen, slightly larger than the iPhone 6s Plus. The Nexus 5X has a 5.2-inch screen.

The debut of the latest Nexus phones underscores the steadily intensifying rivalry between Google and Apple as they duel for consumers’ loyalty in the increasingly important mobile device market. The battle also encompasses tablets and is starting to spill over into the living room as both companies introduce devices that help people watch Internet video on their TVs and listen to online music through their speakers.

Besides its Nexus phones, Google also began selling upgrades to its Chromecast video-streaming device and a new Chromecast model that can be plugged into speakers to play music from a phone or an Internet service. The devices will sell for $35 apiece, the same price that Google has been charging for the original Chromecast, which has won over a lot of households.

Google disclosed Tuesday that more than 20 million of the Chromecast video devices have been sold since its debut two years ago.

Google also provided a peek at a tablet called the Pixel C that is aimed at consumers and workers who want a device that can accommodate a lot of typing. The $499 tablet comes with an attachable $149 keyboard that transforms the device into the equivalent of a laptop, when needed. Apple will begin selling a similar model of its pioneering iPad later this fall.

Apple got a head start on Google with the release of its newest iPhones last week. More than 13 million of the iPhone 6s and iPhone 6s Plus were sold during their first three days on the market, according to Apple, the strongest debut for the device yet.

Although iPhones tend to generate more excitement and longer lines when they are released, far more smartphones run on Android. More than 1.4 billion devices worldwide are now powered by Android, up from 1 billion a year ago, according to Google.