Feb 27 (Reuters) - Belgium's competition watchdog said on Friday it had opened an investigation into the sale of online ads by Google GOOGL.O.

• The competition authorities said they had "serious indications" that Google's model for selling online ads was in breach of antitrust rules, as the company appeared to be abusing its market power.

• The investigation is in a preliminary phase and the outcome is unclear, they said.

• Google is already the subject of several European Union antitrust investigations and has been fined billions of euros in Europe for antitrust violations in recent years.

• Earlier this month, in a letter to advertisers, Google said it also risked another antitrust investigation by the European Commission over concerns it may be unfairly driving online advertising prices higher.

• Google, which dominates the multibillion-dollar online advertising market, said at the time that Google Search ads help small businesses to compete with the biggest brands and keep the web free for everyone.

(Reporting by Bart Meijer. Editing by Mark Potter)

((Bart.Meijer@thomsonreuters.com;))