Amanda Knox told an Italian court Friday that police abused her and called her as a "stupid liar" when they questioned her over the murder of Meredith Kercher.
The American is giving evidence for the first time at her trial in Perugia, where she is charged with sexually assaulting and murdering her roommate.
Knox, 21, said police officers accused her of trying to protect someone after Kercher was found semi-naked with her throat slit in November 2007.
She testified that she had been struck by police during questioning.
"When I denied that they called me a stupid liar," she told the court. "From that point on I was very scared because they were treating me so badly."
Knox claimed the police then pressured her into implicating former bar owner Patrick Lumumba in the murder.
"I didn't expect to be interrogated ...When I got there I was sitting on my own doing my homework when a couple of police officers came and sat with me," Knox testified. "They began to ask me the same questions they'd been asking me all those days ever since it happened — for instance, who could I imagine could be the person who had killed Meredith. I said I still didn't know."
"Everything (I) said was said in confusion and under pressure," she said. "They (the officers questioning me) were suggesting Patrick Lumumba so the first thing I said was 'OK, Patrick.'"
Lumumba's lawyer, Carlo Pacelli, asked her why she had implicated his client during the interrogation.
"They told me try to remember what I apparently, according to them, had forgotten," she said. "I couldn't understand why they were so sure that I was the one who knew everything and so in my confusion I started to imagine that maybe I was traumatized, like what they said. The declarations were taken against my will."
Knox admitted she used marijuana at the time of Kercher's murder.
In reply to a question from a lawyer about whether she took drugs, she said: "Every once in a while, with friends."
Knox is on trial with her former boyfriend Raffaele Sollecito. Both deny any wrongdoing.
The pair previously told police they had taken drugs on the night the 21-year-old was killed and could not remember events clearly because of that.
"On Nov. 1, I told Raffaele that I wanted to watch a movie so we went to his place," Knox said. After dinner, they went upstairs to his room, she said.
"I sat on the bed, he sat at his desk, he prepared the joint and then we smoked it together," the 21-year-old woman said. "First we made love, then we fell asleep."
During her testimony, Knox alternated between English and Italian, occasionally pausing to take a breath, her voice shaky at times.
Knox said she last saw Kercher on the afternoon of Nov. 1. They talked about what they had done the night before — a Halloween night out — and Knox said Kercher still had a bit of her vampire makeup on.
Sollecito then arrived at the house; he and Knox had something to eat while Kercher was in her room, Knox told the court.
"She left her room, said 'bye,' walked out the door," Knox said, at this point speaking Italian. "That was the last time I saw her."
Knox said she and the victim had a good relationship.
"I confided in her, I would often ask for her advice. ... When Meredith had a problem over my behavior, she would tell me," she said. "That was it. There was nothing she would keep hidden or that we couldn't find agreement on."
Sollecito, 25, has said he was at his own apartment the night of Nov. 1, working at his computer. He said he does not remember if Knox spent the whole night with him or just part of it. The two have said they could not remember events clearly because of the drugs.
Knox's father Curt Knox, who is in court, said his daughter was "a bit nervous" but was "100 percent innocent" and was ready for cross questioning by the prosecution because she had "nothing to hide." "Today you will see the real Amanda, not the dark angel described so far" he said.
During the trial, which began in January and is expected to end in the autumn, police witnesses have testified that DNA traces of both Knox and Sollecito were found at the crime scene, and that a kitchen knife found in Sollecito's flat and which is presumed to be the murder weapon had Knox's DNA on the handle and Kercher's on the tip.
Knox and Sollecito have been jailed since shortly after the slaying.
Upon being asked about turning cartwheels in the police station after the killing, Knox replied, "In general I'm someone who tends to act a little silly when I feel I'm in difficulty or not at ease."
They could face Italy's stiffest punishment, life imprisonment, if convicted of murder. The trial began in January and a verdict is expected after a summer break.
A third suspect in the case, Ivory Coast national Rudy Hermann Guede, was found guilty of murder and sexual violence and was sentenced to 30 years in prison. He was given a fast-track trial at his request, and his appeal is set to start in November. He, too, denies wrongdoing.
The Times of London, Sky News and The Associated Press contributed to this report.