The former head of Mexico's national security forces, the center of a controversy that led to the recent firing of the U.S. attorney in San Diego, has been arrested in connection with the continuing investigation of an international car theft ring.

Miguel Nassar Haro, who was director of the police agency for five years and reportedly a valued source of American intelligence officials, was seized in San Diego late Friday after he came to this country to file a libel suit against Time magazine.

Nassar's attorney, Harold Rhoden of Los Angeles, said in a telephone interview yesterday that three FBI agents arrested his client when he walked out of the grand jury room after testifying about the case for nearly two days.

San Diego U.S. Attorney William H. Kennedy was fired by the Reagan administration earlier this month after he told a reporter that his superiors were blocking Nassar's indictment on car theft charges because he was considered such a valuable intelligence source.

At the time, Justice Department sources said no decision had been made on whether to try to prosecute Nassar, so his arrest seems to indicate he will be formally charged.

Officials said yesterday a federal grand jury is to meet in San Diego this week to consider an indictment on car theft and conspiracy charges. An affidavit attached to the arrest complaint charged that Nassar received three stolen cars from the theft ring in 1979.

Rhoden said that the 57-year-old Nassar vehemently denies any involvement in criminal activity. "He came here to clear his name. He wanted to tell everyone he wasn't a car thief . . . . He's not the kind of guy who would drive around in a stolen car. He wouldn't do that any more than J. Edgar Hoover would have when he was head of the FBI."

Investigators have said Nassar was implicated by his own agents, who were among 15 persons convicted of stealing thousands of cars in Southern California and driving them across the border. Several vehicles were traced to a parking lot of the federal security forces.

Investigators said the Mexican security agents decided to take control of the multimillion-dollar ring after first investigating it. Many of the stolen cars went to Mexican government officials and none was ever returned to the United States, the court records allege.

Nassar held a press conference in Los Angeles last Wednesday to announce his suit against Time and was promptly subpoenaed before the San Diego grand jury. He is being held in jail in lieu of a $200,000 bond.

Nassar told reporters Thursday that he helped the U.S. government on criminal matters, but he denied being a paid agent for the Central Intelligence Agency. Rhoden said his client had helped U.S. Embassy officials in Mexico City, but didn't know if any of them were intelligence agents.

"They were never announced as CIA. He was never paid, never an informant," Rhoden said. He said Nassar had helped rescue Americans who had been kidnaped by terrorists in Mexico and had received a medal for rescuing a Belgian diplomat.