The son of Mobil Oil Corp. President William P. Tavoulareas vehemently denied yesterday that his father set him up in a London-based shipping management firm or used his influence with Mobil to help his son's business venture prosper.

Testifying in his and his father's $50 million libel suit against The Washington Post, Peter W. Tavoulareas told a six-member federal court jury here that two Post articles stating his father helped him in business had embarrassed him and damaged his credibility in the Greek shipping community.

After the articles, published Nov. 30 and Dec. 1, 1979, Tavoulareas, now 32, said he was the subject of rumors and was once detained and searched by U.S. Customs. The publicity, he said, was even worse for his father.

"I perceived that this exceptional man was being ridiculed, being called a thief, a liar, a perjurer. . .at the same time I was being called a rogue and a playboy," Tavoulareas testified.

Looking frequently at the jury and striking a conversational tone as he tried to explain the intricacies of the shipping industry, Tavoulareas said that he has an extremely close relationship with his family and speaks to his father "every day of my life."

Indeed, he said, growing up with his Greek ancestry and among his father's business and social friends made getting into the shipping industry "a natural for me."

But Tavoulareas testified that his father had not arranged for him to get his first shipping job, with C.M. Lemos & Co. Ltd. Nor, he testified, had the elder Tavoulareas subsequently used his influence to set his son up with the Atlas Maritime Co. Atlas managed shipping operations for the Saudi Maritime Co. (Samarco) in which Mobil was a partner.

Tavoulareas testified that following his 1972 graduation from Columbia University, where he earned an MBA in international business, he met with the head of the Lemos firm in Greece and applied for a job on his own. He was soon hired in the London office. There, he said, he met George Comnas, and developed such a close relationship with him that Comnas urged him to join forces in business.

By the time he joined Comnas, then a senior partner in Atlas, Tavoulareas said his father had already checked with Mobil's conflict-of-interest committee and had been told there would be nothing improper about the younger Tavoulareas going to work for Atlas.

Tavoulareas said Comnas was dismissed from Atlas in 1975 after a dispute over a business deal in Japan. Peter Tavoulareas and another partner in Atlas were left to run the firm.

In testifying that he had obtained work and prospered in the shipping industry because of his own efforts, Tavoulareas said The Post articles called his integrity into doubt and hurt his ability to charter out his ships.

"Are you going to want to charter one of my ships when it's been written that I'm incompetent, that I'm a crook?" he asked the jury.

Cross-examination of Tavoulareas was recessed yesterday to give U.S. District Judge Oliver Gasch time to hear new arguments on whether the senior Tavoulareas should be treated as a public or private figure for purposes of the libel case. The judge had ruled earlier that the Mobil president should be considered a public figure, but Gasch has reopened arguments on the subject. Lawyers for both parties will conclude their presentations on that issue tomorrow.