Microsoft Corp is offering customers of Salesforce.com Inc thousands of dollars in rebates to switch over to its own customer relationship management system.

In an advertisement placed in the West Coast edition of the Wall Street Journal on Monday -- timed to coincide with the start of Salesforce.com's annual 'Dreamforce' user conference in San Francisco -- Microsoft is offering companies $200 per user to move to its product.

The incentive, to be used as a rebate on the cost of transferring to Microsoft's system, could add up to tens of thousands of dollars or more for some companies.

Microsoft's move is the latest in a lively rivalry between the two companies, vying over the emerging role of cloud computing, or providing computing power and applications over the Internet.

Salesforce.com, which sells programs to organize companies' contacts with its customers, is a leader in cloud and aggressively campaigns against the use of installed software, which is Microsoft's stock in trade.

Microsoft has countered by taking many of its services online and now offers alternatives to Salesforce.com's products using a combination of installed software and databases accessed online.

At Microsoft, we do not believe you should be forced to pay a premium to achieve business success, Microsoft executive Michael Park wrote in an open letter to Salesforce.com customers, which appeared as an ad in the Journal on Monday.

In the letter, he urged customers at the Dreamforce conference to ask Salesforce.com to justify its prices and service. Salesforce.com had no immediate comment.

Salesforce.com Chief Executive Marc Benioff is a strident critic of Microsoft, which he has referred to as a technology monopolizer and patent troll.

Earlier this year, the two companies settled a patent disagreement over who owns the technology behind their respective systems.

Microsoft shares, which are down 12 percent this year, fell 0.7 percent in afternoon trading on Nasdaq. Salesforce.com shares, which have almost doubled this year, were up 1.25 percent on the New York Stock Exchange.

(Reporting by Bill Rigby; editing by Andre Grenon)

Marc Benioff Marketing Microsoft Salesforce