WASHINGTON (Reuters) - U.S. prosecutors sought on Tuesday to protect their star witness in the upcoming perjury trial of former major league pitching star Roger Clemens.

The Justice Department asked a judge to restrict defense lawyers from asking Brian McNamee, Clemens's former trainer, about a past run-in with the law.

Clemens is due to go on trial on July 6 on six counts related to allegations he lied during a congressional investigation into sports figures using performance-enhancing drugs.

McNamee is expected to testify that he injected Clemens with performance-enhancing steroids and human growth hormone several times.

Clemens, who won more than 350 major league games over 24 seasons, has denied taking the drugs or lying about it.

Prosecutors filed a motion seeking to restrict Clemens's lawyers from asking McNamee for details about a 2001 incident in which he lied to Florida police investigating allegations of a sexual assault.

They said Clemens's lawyers, who would likely want to try to undermine McNamee's credibility, should only be allowed to ask whether McNamee lied to police during an investigation and no more.

"In short, the defense may impeach Mr. McNamee's character for truthfulness by asking whether he made false statements to Florida police in 2001," prosecutors said in the motion.

"They may not, however, inflame the jury (and pursue irrelevant paths) through evidence or argument about Mr. McNamee's investigation, or about long-off, uncharged allegations of sexual assault," they said.

Lawyers for Clemens and McNamee could not be reached for comment.

Clemens's attorneys also filed their own request on Tuesday seeking to bar some testimony by Clemens's former teammates which prosecutors plan to call as witnesses, arguing they have no direct evidence about Clemens's alleged use of the drugs.

The other players included Andy Pettitte, Chuck Knoblauch and Mike Stanton.

To allow them to testify about their drug usage and ties to McNamee would unfairly allow prosecutors to suggest "guilt by association" and boost the trainer's credibility, Clemens's lawyers said.

The case is USA v. William R. Clemens, No. 10-cr-223 in U.S. District Court for the District of Columbia.

(Reporting by Jeremy Pelofsky; editing by Todd Eastham)