Japanese electronics and entertainment giant Sony on Tuesday unveiled its first tablet computers, codenamed S1 and S2, in a direct but belated challenge to Apple's iPad.

The larger "Sony Tablet" S1 has a single screen while the pocketable S2 has twin screens, company officials told a news conference, with both devices using Google's Android operating system and equipped with Wi-Fi for Internet access.

Competitors have rushed to cash in on soaring demand for tablets since the iPad was released in April last year, but Sony's devices are not due to go on sale globally until the northern hemisphere autumn, well behind its rivals.

Research in Motion is the latest to join the fray, with the release last week of its Blackberry PlayBook.

Sony said earlier this year it planned to be the number-two tablet maker by 2012 but until now had given little indication of how it intended to compete in a market already dominated by the iPad.

Samsung's Galaxy Tab is the best-selling rival to the Apple gadget but technology research company Gartner says iPad will keep its crown for the next few years despite competition in an expanding market that has eaten into PC sales.

The iPad accounted for 83.9 percent of the total 17.6 million tablets sold in 2010, according to Gartner, which predicts worldwide tablet sales will soar to 294.3 million in 2015.

Unveiling its first tablets, Sony said they would have access to online content to buy and download videos, music, digital books and other entertainment and be compatible with existing PlayStation games.

"Developers generally tend to add lots of features, but we mostly thought about what the user really wants and needs," Sony official Kunimasa Suzuki said.

The S1 looks like a direct competitor to the iPad, with a 9.4 inch (24 centimetre) screen, and front and rear cameras while the clamshell S2 recalls Nintendo's DS portable console but with dual 5.5 inch colour touchscreens.

"This design is particularly relevant for reading digital books whose content is displayed on screen as two pages side-by side," Suzuki said.

Both screens can be used together as a single large screen or for playing games on one and displaying control buttons on the other.

The S1 can also work as a universal remote to control audio-visual equipment or send content to television screens or music to wireless speakers, Sony said.

The two devices use Google's Android 3.0 operating system, known as Honeycomb, which is optimised for devices with larger screen sizes.

"I'm excited about 'Sony Tablet' as it will further spur the development of applications and network offerings, which users are looking for," said Andy Rubin, senior vice president of Google's mobile division.

The announcement comes as Sony looks to focus more on pushing its content such as games and music through hardware platforms including game consoles, smartphones and tablet computers.

The company did not give any indication of pricing.

Sony also announced a new line of "hybrid" notebook computers that feature a slide screen covering a keyboard.

Sony shares closed down 2.11 percent at 2,415 yen in Tokyo.