DALLAS—Opening arguments in the insider trading trial of billionaire investor Mark Cuban began here Tuesday with government lawyers portraying the defendant as a ferocious competitor who, with an unfair advantage over the investing public, sold shares illegally.

"Insider trading is a form of cheating," said Securities and Exchange Commission lawyer Jan Folena as the trial got under way. "It's like being the only person in the class who knows the answer to the test before you take it."

Mr. Cuban, the owner of the National Basketball Association's Dallas Mavericks, was in turn depicted by his lead lawyer as innocent and willing to stand before the government because he has nothing to hide.

"The guilty man flees when one gives chase but the righteous man stands bold," Thomas Melsheimer, a lawyer for Mr. Cuban, told the jury in his opening argument.

The long-running case stems from a suit the SEC filed in 2008, claiming Mr. Cuban violated federal securities laws in 2004 when he sold his entire 6% stake in a Canadian Internet search company after learning inside information from the company's chief executive.