A trial pitting the Securities and Exchange Commission against Marc Belzberg and First City Financial Corp. began yesterday in federal court, with the SEC's ability to prosecute alleged takeover abuses successfully also on trial.

The stakes in the case go beyond the immediate question of whether Belzberg and First City violated securities laws by concealing ownership of more than 5 percent of Ashland Oil Co. stock in March 1986. With the SEC in the midst of major investigations of Drexel Burnham Lambert Inc. and others, the case has become a test of whether the SEC can win complex cases against defendants who contest charges rather than cooperate with authorities.

Belzberg's attorney, Arthur Liman, who served as chief counsel of the Senate committee investigating the Iran-contra affair, also is representing Drexel Burnham junk bond specialist Michael Milken in the SEC investigations. The Drexel investigation and the Belzberg case involve questions of stock trading and disclosure in connection with corporate takeovers.

Marc Belzberg and his father, First City Chairman Samuel Belzberg, sat side by side in the front row of Judge Barrington Parker's courtroom yesterday. Both men are expected to be witnesses in the case. Samuel Belzberg, who directs the family's multibillion-dollar empire from Vancouver, said it was his first time ever inside a U.S. courtroom; he spent much of the proceeding yesterday rubbing and twirling a flat blue stone, which he regularly removed from his suit coat pocket.

The SEC has charged that prior to making a takeover bid for Ashland Oil last year, the Belzbergs concealed their ownership of more than 5 percent of Ashland by "parking" stock with the Wall Street firm of Bear, Stearns & Co.

Once the Belzberg's interest in Ashland or any other public company becomes public knowledge, the stock price of the potential takeover target typically goes up. The SEC charged that Marc Belzberg and First City improperly concealed their substantial Ashland holdings by purchasing shares through Bear Stearns and leaving the shares in a Bear Stearns account, enabling them to continue buying Ashland stock at artificially low prices. SEC rule 13d requires investors to disclose purchases of 5 percent or more in public companies within 10 days.

In his opening argument yesterday, SEC attorney Barry Goldsmith asked Judge Parker to make the Belzbergs give up about $2.7 million in stock trading profits on shares purchased after they allegedly failed to meet disclosure requirements. Goldsmith said that Bear Stearns Chairman Alan C. (Ace) Greenberg, who purchased the shares in question on the Belzberg's behalf, has told the SEC he thought he was purchasing the shares for the Belzbergs.

But Liman and Max Gitter, the attorneys from the law firm of Paul, Weiss, Rifkind, Wharton & Garrison who represented the Belzbergs, said there was a misunderstanding involving Greenberg and Marc Belzberg rather than a securities law violation. Liman said Marc Belzberg never directed Greenberg to purchase the Ashland shares for the Belzbergs; Gitter said Marc Belzberg never received the necessary permission to direct such purchases from his father.

After making a $1.7 billion hostile takeover bid for Ashland last year, the Belzbergs ultimately made a $15.4 million profit when they sold their Ashland shares back to the company and dropped the bid.

Also yesterday, Securities and Exchange Commission Chairman David S. Ruder said commission investigations arising out of the cooperation of former stock speculator Ivan F. Boesky and others are "months rather than weeks" away from completion.