Microsoft Corp.'s first-ever homegrown computer has created confusing new battle lines in the tech sector, turning longtime allies into rivals and underscoring a philosophical split over the purpose and future of tablet-style devices.

The Surface tablet, unveiled by Microsoft Monday, has an unusual detachable keyboard that makes it a standout rival to Apple Inc.'s iPad and conventional laptop computers as well.

Microsoft's tablet alters its longtime role as software supplier to computer makers. But more broadly the move underscores a debate with Apple over what a tablet should be: a new category of consumer devices, or a traditional PC in a slimmer, on-the-go package.

Steve Ballmer, Microsoft's chief executive, argued Monday that tablet users want to be able to use their gadgets both for passive activities such as watching a movie, and for creating spreadsheets or other workplace functions. The Surface, to address those goals, includes screen covers that double as clip-on keyboards.

'They want to be on their couch, they want to be at their desk and they want to be on the go,' Mr. Ballmer said. 'People want to do all of that without compromising the productivity that PCs are uniquely known for.'

His company, and others such as Intel Corp., have projected that differences between tablets and laptops will fade as the latter add touch-screen capabilities and more convertible designs with keyboards emerge.

Apple, by contrast, has consistently argued tablets are entirely separate from portable PCs. 'If you force them together, I think the PC is not as good as it can be, and I think the tablet is not as good as it can be,' said Tim Cook, Apple's chief executive, at the D: All Things Digital conference last month.

The Cupertino, Calif., company speaks with some authority, given its extraordinary success in creating a new market with the iPad in 2010. Apple is expected to be responsible for nearly two-thirds of the 107.4 million tablet computers units research firm IDC expects to be shipped this year, while devices running Google Inc.'s Android operating system are estimated to take a 36.5% market share.

The iPad and other tablets are already eating into demand for laptop computers in industrialized countries, particularly for low-end models called netbooks aimed at simple Web tasks. Research firm Gartner Inc. is forecasting unit PC sales will increase just 2.7% in 2012, while tablet sales are predicted to nearly double this year to nearly 119 million units.

Intel has attempted to generate more excitement by encouraging the creation of sleeker portables called Ultrabooks, which include convertible models that can be used like tablets. But sales have been slow so far.

The Surface, with its detachable keyboards, is another alternative that could pose additional competition for laptop makers. 'With the Surface, Microsoft is promoting versatility, and really directly competing with Ultrabooks,' said Forrester Research Inc. analyst Sarah Rotman Epps.

Where Microsoft sets pricing for the Surface--particularly in relation to the iPad, which starts at $499--could ultimately determine its true competitor.

The Surface comes in two versions. Microsoft said the model using Windows RT--a variant of the Windows 8 operating system--and chips from Nvidia Corp. should arrive first and be in line with pricing of similar tablets.

A more powerful version running Windows 8 and chips from longtime partner Intel will start closer to the pricing of Ultrabooks, which often cost $900 or more.

Patrick Moorhead, a market researcher with Moor Insights & Strategy, said Microsoft may try to ease relations with computer makers by pricing Surface $100 or so higher than their offerings. But that could hobble its own sales efforts, he said.

Mr. Moorhead predicted some hardware companies that had been planning to release tablets based on the new versions of Windows may change their minds. 'With Surface, the dynamic between Microsoft and its customers changes, forever,' he wrote in a research note.

Some of Microsoft's hardware partners were indeed annoyed by the announcement, since Surface could compete with their products, people familiar with the matter said. The development project was kept closely guarded, even among Microsoft executives.

Some executives at chip and computer system makers weren't told about Surface until days before the launch event, and even then were informed about the project only in broad strokes, according to people familiar with the matter.

Hardware companies Hewlett-Packard Co., HTC Corp. and Acer Inc. declined to comment on their relationships with Microsoft. Lenovo Group Ltd. and Dell Inc. said they remain committed to releasing a lineup of tablet-style computers powered by Windows software. A Lenovo spokesman said the company is 'in a very strong position to compete and win in the tablet market, against all comers.'

A Microsoft spokeswoman declined to comment. But Microsoft executives Monday sought to play down tensions with its hardware allies. Mr. Ballmer said the company's relationships with hardware companies are 'essential' to Windows 8 and Windows RT.

'We believe in the strength of that ecosystem, of software and hardware companies that work together to deliver selection and choice,' Mr. Ballmer said.

Some Microsoft allies argue that Surface could help spur innovation, by setting expectations for what new tablets computers should be. 'It's a great thing for the marketplace to see competition around a great design,' said Rob Chandhok, who heads software strategy for chip maker Qualcomm Inc.

Shira Ovide / Don Clark