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LOWER MANHATTAN — Emotions ran feverishly high as a mother encountered the nanny who killed her two children for the first time since their deaths six years ago.

Thursday was day one of the murder trial of nanny Yoselyn Ortega, 55. The first witness in the trial was Marina Krim, the now 41-year-old mother of the victims. She had discovered the bloodied bodies of her daughter Lulu, 6, and son Leo, 2, in the bathroom of her family’s apartment in October 2012.

“What she saw was unthinkable,” Assistant District Attorney Courtney Groves in her opening statement said. “Bloody bodies, eyes open, covered in blood, staring ahead.”

Prosecutors argued that Ortega was fully aware that she was committing a heinous crime when she’d done it, in contrast to claims that the former nanny has since made, alleging that she was demon-possessed at the moment of the killings.

“The defense in this case is ‘the devil made me do it,'” Prosecutor Groves said, critically.

She added that the prosecution’s evidence will prove one thing.

“It will be clear to you that she’s guilty and she’s responsible for her actions,” Groves said

In contrast, Ortega’s defense attorneys argued that their client’s mental state prevented her from understanding what she was doing at the time of the killings. Defense attorney Valerie Van Leer-Greenberg had tried, for years, to get the judge to conclude that Ortega was mentally unfit to stand trial. The judge was ultimately not persuaded, but it was still a key component of Van Leer-Greenberg’s opening statement.

“I will show that the lack of motive in this case is the hallmark of the mentally ill defendant,” she said.

As the murdered children’s mother, Marina Krim, headed to the stand to testify, she was clearly emotionally distraught.

“I’m sorry, I just need a good look, you guys,” she said, as she entered the witness stand.

It was the first time she’d been face to face with Ortega since the Oct. 25, 2012 killing.

“You are a liar!” she exclaimed, looking right at the woman to whom she’d entrusted her children.

Then Krim sat down, and began nearly four hours of testimony about the worst day of her life.

“It’s like a total horror movie,” she said, choking back tears, and breathing heavily, as she described finding her daughter’s and son’s bloodied bodies. Her daughter had been stabbed 30 times, and her son sustained six stab wounds. The injuries were so severe, that responding paramedics had thought the two had been decapitated.

“She killed my best friends,” Krim testified. “I don’t know how else to say it.”

Krim is expected to be questioned by defense attorneys on Friday.