BRUSSELS - Oracle Corp's (ORCL.O) customers told concerned European Union regulators on Thursday that the U.S. software company's $7 billion deal to buy Sun Microsystems Inc (JAVA.O) would not curtail competition in the database market.

Oracle, the world's biggest database software maker, said customers uniformly and consistently supported its case during the first day of a closed hearing held by the European Commission which has objected to the deal over competition concerns.

I am extremely happy. There can't be a better voice than customers'. The customers said that there is robust competition in the market, Thomas Vinje, Oracle's lawyer, told reporters.

Commission officials had earlier declined to comment.

Swedish telecommunications network equipment maker Ericsson (ERICb.ST) and British cell phone operator Vodafone Group Plc (VOD.L) were among eight customers expected to attend the hearing on Oracle's behalf.

Oracle President Safra Catz and a representative from the U.S. Department of Justice were present at the event.

Customer support could be critical for the company's case, said antitrust lawyer Silvio Cappellari at Arnold & Porter.

If they have a convincing story to tell, this would certainly increase Oracle's chances to get the deal cleared with little or even no concessions, he said.

Oracle customers such as Spanish bank Banco Bilbao Vizcaya Argentaria SA (BBVA.MC), the British Atomic Weapons Agency, the British National Health Service and Sabre Holdings Corp TSG.UL will present their views on Friday when the hearing continues.

U.S. software company Microsoft Corp (MSFT.O), Germany's SAP AG (SAPG.DE) and Monty Widenius, the creator of Sun's MySQL database, will back the commission's case.

SAP had previously told the commission it was worried that Oracle's acquisition of MySQL would hurt competition in the database market by eliminating one of the top players, according to company spokesman Saswato Das.

SAP has always made it clear that its concern is to secure customer choice in the highly concentrated database market, he said.

While the usefulness of hearings is sometimes questioned, especially by merging parties, there are certainly cases where hearings turned out positive for the merging parties, said Robbert Snelders, a partner at Cleary Gottlieb Steen & Hamilton.

He cited the music joint venture between Japanese consumer electronics company Sony Corp (6758.T) and German publisher and broadcaster Bertelsmann AG BERT.UL that was cleared by the commission in 2004, when a hearing had been useful.

The U.S. Department of Justice has approved the takeover.

(Additional reporting by Jim Finkle in Boston; Editing by Rupert Winchester and Gerald E. McCormick)