Facebook on Thursday unveiled a software suite which stakes out a "home" on Android smartphones as it steps up its challenge to Apple and Google in the booming mobile market.

Chief executive Mark Zuckerberg said the new software weaves the social network into the home screen of HTC and Samsung phones powered by the latest versions of Android to focus experiences on "people and not apps."

"We're not building a phone and we're not building an operating system, but we are building something that's a lot deeper than an app," Zuckerberg told a gathering at the company's headquarters in Silicon Valley.

Facebook called the new software "a new way to turn your Android phone into a great, living, social phone."

The software, which allows users to see Facebook's "Cover Feed" when they turn on their phones, will be available for download from Google's online Play shop in the United States starting April 12, Zuckerberg said.

A version should be available in Europe in coming months, according to Facebook, which said it is in the process of tailoring "Home" for tablet computers.

Home will be available through Taiwan-based electronics firm HTC, which will launch its Facebook home-screen smartphone called HTC First, to be available through US carrier AT&T at a price of $100.

"It is the only phone that has Facebook Home pre-loaded and optimized," HTC chief Peter Chou said while joining Zuckerberg at the event.

"Mobile is fundamentally social; it is all about connecting with other people," Chou continued. "This is a great opportunity to bring mobile and social together even closer."

Zuckerberg demonstrated how a user would start with Facebook on the home screen and be able to navigate and switch back and forth between apps, and simultaneously send and receive messages through "chat heads" which are overlaid on the screen.

"I think this is a really big deal for Facebook," said Peter Corbett of the research firm iStrategyLabs.

"We've actually seen Facebook call themselves a mobile company recently and if you look at their analytics, more than half of their users are accessing Facebook through mobile. Now that they have what will essentially be a native branch of the Android operating system that they control and build upon, this is the future of Facebook."

Corbett added that "Apple should be worried to a certain extent" because Facebook will "put this in front of millions and millions of the world's youth who may decide 'I want a Facebook phone instead of an iPhone.'"

Analyst Trip Chowdhry said the new software is a potentially "groundbreaking experience" which could help Facebook compete with Google in the market for mobile advertising.

"Since Facebook is not imitating, they will carve a leadership position," he said, adding that "Google missed a huge opportunity in social mobile phones."

Facebook said no ads will be woven into social network feeds that appear automatically as home screen or lock-screen scenes but that they may in the future.

Facebook customized Home for Android-powered smartphones because the operating system made available free by Google can be openly tweaked by hardware makers as opposed to the tight grip Apple keeps on iPhone innards.

Zuckerberg was careful not to throw down overt challenges to Apple or Google.

"We have a great relationship with Apple," Zuckerberg said.

"Google is aware of what we are doing; we have talked to them," he added. "We are committed to doing our best on every platform."

Zuckerberg reasoned that since people spend an average of 20 percent of their smartphone time at Facebook, improving the experience on Android handsets was a good thing for Google's mobile operating system.

"Every company that I know wakes up in the morning trying to build the best experience; and Google does too," Zuckerberg said.

"In a way, this can start to bring some of those high quality experiences you see on iPhones to Android, and that could be really good for Android."

The announcement comes with Facebook trying to connect more with mobile users, and -- importantly -- deliver more ads in the fast-growing segment.

The research firm eMarketer said US mobile advertising spending grew 178 percent last year to $4.11 billion, a market that nearly tripled last year.

Google took more than half of those revenues, but Facebook's share in growing: eMarketer said the social network accounted for 9.5 percent of mobile ad revenues in 2012 and is expected to take 13.2 percent this year.