REDWOOD CITY, Calif. – Scott Peterson’s lawyer yesterday brutally portrayed his client’s ex-mistress as a woman scorned who would stop at nothing to try to help cops nail him in his wife’s murder – even by lying to him that she was pregnant with his baby.

Lawyer Mark Geragos also blasted sexy blond masseuse Amber Frey on the witness stand by suggesting she’s simply a desperate, jealousy-driven, drunken floozy.

“You told them [Modesto cops] you had sex with Scott on at least three occasions out of the four or five times you were with him, and that one of these times you had unprotected sex, right?” Geragos asked.

“At that point, I was willing or open . . . to assisting the police any way I could help,” Frey responded.

The exchange came as newly released police reports revealed details of Frey’s conversation with cops Dec. 30, 2002 – six days after Peterson’s pregnant wife, Laci, was reported missing from their Modesto home.

“He [Peterson] doesn’t know that I’ve had my period,” Frey told the cops.

“So you’re suggesting that we could probably use that as a ruse or something like that?” one cop said.

“Yes, oh yeah,” Frey said.

It appears she never wound up going through with the plan.

In trying to paint Frey as a loose woman, Geragos also recalled details of her and Peterson’s early dates, which ended with hot sex.

“You were drinking straight [through the night] once you met this guy?” Geragos asked Frey of her and Peterson’s first date in November 2002.

“Yes,” the 31-year-old single mom replied stoically.

Minutes later, Geragos asked, “You spent the night together [on a second date less than two weeks later], and then the following morning he took off?”

“Yes,” admitted the prosecution’s star witness.

Geragos even brought up the fact that Frey had left her 2-year-old daughter with Shawn Sibley – the pal who introduced her to Peterson – so she could meet Peterson.

“She [Sibley] was going to sleep overnight just in case you didn’t come home?” Geragos asked.

The lawyer also suggested that Sibley had told Frey the nickname that Peterson’s friends had given him – “Horny Bastard.”

But Frey said she couldn’t remember Sibley telling her about the moniker.

Frey – her dyed-blond hair neatly parted down the middle – donned a conservative gray skirt and jacket for the much-anticipated first day of her cross-examination by Geragos. She appeared calm and emotionless as she answered each question.

Meanwhile, jurors intently studied her demeanor and responses on the stand, then quickly jotted down notes.

Frey’s testimony is considered crucial to the prosecution’s contention that Peterson coldbloodedly killed eight-months-pregnant Laci and then dumped her body in San Francisco Bay. The bodies of Laci and their unborn fetus washed up on shore in April 2003.

Frey turned to the cops shortly after Laci’s disappearance, when, she says, she first discovered Peterson had lied to her about being married.

Prosecutors say Peterson’s obsession with Frey was one of the reasons he wanted to rid himself of his other life with a wife and soon-to-be-born child.

But Geragos painted Frey as being the one who was crazy about Peterson – and wasn’t about to let him out of her clutches.

The lawyer presented the Christmas present that Peterson had sent Frey in 2002: a “Star Theater 2,” which projects astral patterns across a darkened room and retails for only $34.95, asking:

“This is the Christmas present? This is what he sent you? That’s why you called him 14 times [on Christmas], because of this?”

“Yes,” Frey said.

At the start of yesterday’s cross-examination, Geragos tried to open on a light note – and even brought snickers from some jurors.

When the judge told him he could begin, Geragos said, “No questions.”

As the judge’s jaw dropped, the lawyer quickly added with a smirk, “Just kidding.”