Amanda Knox, the American student charged with the murder and sexual assault of Meredith Kercher, faces an additional charge of slander for claiming that police struck her while she was being questioned.

At the latest hearings in her trial in Perugia, Ms Knox claimed that police had put her under psychological and physical pressure to admit that she was present at the murder.

Ms Knox, who has the right to address the court at any time during her trial, was reacting to evidence from Anna Donnino, a police interpreter who claimed that Ms Knox had behaved "as if a weight had been lifted from her" when she admitted that she had been at the scene of the crime and accused Patrick Diya Lumumba, a Congolese bar owner for whom she worked part-time, of the killing. Ms Knox told police that she had covered her ears in the kitchen to block out Ms Kercher's screams.

Ms Donnino said that when questioned after Ms Kercher's body was found, Ms Knox walked up and down nervously at the police station, "hitting her head with her hands". She had denied responding to an SMS message from Mr Lumumba telling her there was no need to come to work because there were few customers, leaving her free for the evening. But she broke down when police said phone records showed that she had done so, Ms Donnino said.

"She showed extreme emotional involvement – she was crying and visibly shocked, saying 'It was him, it was him. He's bad'," Ms Donnino added.

Ms Knox, speaking in fluent Italian, said police had called her a "stupid liar" during "hours and hours" of questioning during which she had stuck to her story that she spent the night of the murder at the flat of Raffaele Sollecito, her former boyfriend and co-accused.

She said that Ms Donnino had suggested to her "that probably I didn't remember well because I was traumatised, so I should try to remember something else". There had been an "aggressive insistence" on the text message she had received from Mr Lumumba, Ms Knox said. She insisted she had been slapped on the head by police, adding "I'm sorry, but it's true".

Ms Donnino said that Ms Knox had been "comforted" by police, given food and drink, and had at no stage been hit or threatened.

The newspaper Corriere dell' Umbria said that Giuliano Mignini, the prosecutor, would bring an additional charge of slander against Ms Knox, since all police officers and interpreters who have given evidence at the trial have testified under oath that she was at no stage put under pressure or physically mistreated.

Ms Kercher's semi-naked body was found under a duvet on the floor of her bedroom in November 2007, at the hillside cottage in Perugia she shared with Ms Knox and two Italian women. She had been stabbed in the throat.

The prosecution accuses Ms Knox and Mr Sollecito of murdering and sexually assaulting Ms Kercher with Rudy Guede, an Ivory Coast immigrant who was given a 30-year sentence last October for the crime under fast-track procedures. He began his appeal last week, claiming Ms Knox had killed Ms Kercher in a row over stolen cash.

The all-night interrogation in which Ms Knox accused Mr Lumumba and described blocking her ears was ruled inadmissible by Italy's Supreme Court because no lawyer was present. However a voluntary statement written by Ms Knox in English repeating this scenario has been accepted as court evidence despite defence protests. The defence claims Ms Knox was not at the cottage during the murder but at Mr Sollecito's flat.

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