Clarence Mixon, who once owned a Portsmouth, Va., car dealership with former state senator Willard J. Moody, today portrayed Moody and his political protege, state Sen. Johnny S. Joannou, as active participants in an alleged scheme to fix traffic tickets for their friends, relatives and political associates.

Moody and Joannou are standing trial in U.S. District Court here on charges in the alleged scam. Moody is charged with eight counts of perjury and obstruction of justice. Joannou, a Portsmouth Democrat now serving his first term as a state senator, is charged with one count of conspiracy and 12 counts of mail fraud.

Mixon, who was described Monday as a "liar and a thief" by Wayne Lustig, Moody's attorney, is the most unpopular character in the trial of the two Portsmouth lawyers, now in its second day. He is also the prosecution's most important witness.

The trial may turn on whether Mixon, now serving an 18-month sentence for wire fraud, is telling the truth when he says Moody and Joannou conceived and helped orchestrate the production of phony documents from Sun Motor Cars Inc. in order to help their friends, relatives and law clients get their speeding charges reduced or dismissed.

Mixon, 50, is expected to be the only witness for the prosecution to directly link Moody and Joannou to the ticket-fixing scheme.

It is an accepted fact in the trial that the dealership produced phony documents purporting to show faulty speedometers during the six-year period ending in 1982.

Mixon testified today that Joannou approached him about producing the certificates after opportunities to fix speeding tickets in Portsmouth died in the mid-'70s with the indictment of several top Portsmouth police officers.

"We've got to find a way to do these tickets," Mixon said Joannou told him.

Mixon said Moody instructed him to "go ahead and do it, but don't do it for anybody else but Johnny." In return, Mixon said, Joannou promised to represent his relatives in court free of charge.

Over the years, Mixon said he ordered his employes to write up phony documents for a long list of and political cronies of Moody and Joannou people, including Moody himself after he was caught driving 68 mph in a 55 mph zone.

Federal prosecutors allege that Moody sent Mixon a note with his ticket that said, "Guess what? . . . I wasn't paying attention."

In most cases, Sun Motor employes said they drew up what U.S. District Judge Richard L. Williams called the "15 miles-per-hour special," showing that speedometers that had never been checked were off by 15 miles an hour.

Joannou, 44, would bring "four or five" speeding tickets at a time to have false calibration forms prepared, Mixon said.

Mixon said Moody kept close tabs on the operation, advising him to alternate Sun Motor forms with those of a second bogus firm because judges were becoming upset about the number of Sun Motor certificates.

Later, Mixon said Moody, fearful of an FBI investigation, told him to burn his copies of the certificates, saying "I burned mine."

Joannou, according to Mixon, was less concerned. He said Joannou told him, "If you're not worried about it, I'm not either."

Mixon said Moody, 60, was a knowing participant in a range of illegal endeavors extending well beyond the ticket scam, but did not elaborate on those activities.

Mixon testified that he had lied repeatedly under oath about the scam but decided to tell the truth after federal authorities threatened to have him jailed indefinitely for contempt of court.

Defense attorneys promised a "vigorous cross-examination" of Mixon Wednesday. Under questioning by the defense this morning, three former Sun Motor employes who said they prepared phony certificates, testified that they had no reason to believe either Moody or Joannou knew of the scam.