BRUSSELS (Reuters) - Microsoft and SAP will voice their concerns at a European Union hearing on Friday over Oracle's plan to buy Sun Microsystems, adding to regulators' skepticism over the deal.

The European Commission has objected to Oracle's $7 billion bid for Sun, saying the purchase may limit competition in the database market. The EU competition watchdog has until January 27 to decide on the deal, which its U.S. counterpart has cleared.

Software giant Microsoft's representatives and lawyers declined to comment before the two-day hearing, which started on Thursday.

German software company SAP, whose chief rival is Oracle, has told the Commission Oracle's acquisition of Sun's MySQL database could hurt competition in the market by eliminating one of the top players.

Oracle, the world's biggest database software maker, said its customers told Commission officials and European national regulators at the hearing on Thursday that the takeover was not anti-competitive.

Oracle lawyer Thomas Vinje told reporters the Commission had misinterpreted the content of its file and that respondents to the Commission's market test and survey had overwhelmingly supported the company's bid.

Monty Widenius, creator of MySQL, will back the Commission's case on Friday while Swedish telecoms network equipment maker Ericsson will argue for Oracle.

Antitrust lawyers said enlisting major companies to express support at a hearing could sway doubtful regulators.

(Reporting by Foo Yun Chee, editing by Dale Hudson)