OAKLAND -- NCAA president Mark Emmert on Thursday offered a full-throated defense of the amateur status of college athletes, insisting that any form of paying student football and basketball players would destroy the popularity of college sports.

Serving as the NCAA's chief witness in a test of the organization's rules against compensating student athletes, Emmert told Chief U.S. District Judge Claudia Wilken that it is crucial to maintain a distinction between amateur college athletes and professional athletes paid to do a job.

"They are full-time students who play their sport at their university," Emmert testified, seeking to refute the claim that academics take a back seat for student athletes. "It is one of the core fundamental principles of the NCAA."

Emmert was pressed repeatedly under cross-examination about the commercialization of big-time college sports. He was testifying in the second week of trial in a class-action lawsuit brought on behalf of former Division I football and basketball players seeking the right to share in the multibillion-dollar college sports empire. It marked the first time an NCAA president has been hauled into a trial to defend the status quo in college sports.

The trial could dramatically change the landscape of college sports, depending on Wilken's decision. The former athletes argue that the NCAA's rules violate federal antitrust laws, and they want the court to block enforcement of limits on compensation of college football and basketball players.

A "pay to play" system would undermine college sports, warned Emmert, who took over as the NCAA's chief in 2010. He testified that college sports would lose a large share of audience if players were paid like professionals and that some universities would pull out of Division I sports altogether.

Despite the explosion of television dollars for college football and basketball, Emmert told the judge, most college sports programs do not make much money because schools gobble up the revenue to pay for expenses such as scholarships, coaches' salaries, travel and facilities for all 16 Division 1 sports.

His testimony was designed to show that adding the cost of paying athletes beyond their scholarships would cause hardship to overall sports programs. The former athletes are arguing that they should get a share of television, merchandising and video game revenue because all of them use the athletes' names and images.

"If you took away all that revenue, (college sports programs) simply would not operate as they do now," Emmert said.