Click here for a slideshow of Android prototypes and other phones

Owners of the new Google-powered mobile phone will be able to unlock the handset by drawing a secret shape on the screen.

The new 'signature unlocking' tool was among the features revealed during a sneak preview in California yesterday.

Other highlights include a built-in compass that will allow people to orientate maps as they use their phone to scout out a restaurant or venue, and a customisable homepage that lets people bookmark their favourite web pages.

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The device - which is unlocked by drawing a shape only the owner knows on a nine-square grid - will also include a magnifying tool, to make zooming in on web content easier on a small screen, and a mobile version of the game Pac Man.

Demonstrating the device at a developers' conference in San Francisco, Andy Rubin, who heads up the project at Google, declined to give a release date, but said that the first phones powered by Google's Android operating system will appear in the second half of the year.

Google will not make the phone, but has helped develop the software that handset manufacturers will install in their devices. Samsung, HTC, LG Electronics, and Motorola are among the companies that have said they will produce phones that run on Android.

The device on which Mr Rubin gave the demonstration (a video is here) had a touch-sensitive screen, but the software will work equally well on other devices, he said, including those with a so-called 'tracking ball', which has been used by BlackBerry.

Observers of the demonstration said the software bore a resemblance to that used on Apple's iPhone, which is also a touchscreen device, and which allows owners to place icons linking to sites such as YouTube on the homepage.

Google demonstrated the device to about 3,000 software developers at an annual conference, and said that it hoped developers would create all kinds of applications that owners of Android phones will be able to download from the internet and install on their devices.

Android is what is known as an 'open-source' operating system, meaning that developers can access the code and create software that works with the device. Apple has announced a similar inititiave which allows developers to create software for the iPhone.

Google, which handles about 80 per cent of search queries in the UK, also hopes that by helping to produce a phone that will make it easier to use the web, it will tap a new source of revenue - namely advertisements that appear on web pages viewed on mobile phones.

The search company reported revenues of just over $5 billion in the last quarter, but the vast majority came from adverts viewed on personal computers. In Western Europe, the spend on mobile advertising is expected to rise from $1 billion in 2008 to $1.5 billion this year.

In a bid to take on Google in mobile, Microsoft announced last week that users of its e-mail and messaging tools on mobile phones would for the first time see ads on such services.

According to a M:Metrics, a company which tracks use of the mobile internet, 62 per cent of search queries by UK mobiles are performed by Google, compared with 7 per cent by MIcrosoft.

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