The celebrity chef appeared at Isleworth Crown Court on Wednesday morning to give evidence against her former personal assistants Elisabetta and Francesca Grillo, who stand accused of fraud.
The Italian sisters allegedly spent nearly £700,000 on luxury goods using credit cards.
They deny the charge, which is said to have taken place between January 2008 and last December, and have claimed that they were allowed to use the cards in exchange for covering up Nigella's drug use.
Nigella categorically denied their allegations. She said her court appearance came after a "long summer of bullying and abuse", and that following the now infamous "awful incident at Scotts (restaurant)", false allegations of drug use began circulating on a "PR blog".
Charles arriving at court 29 November
She claimed that these allegations were "dedicated to salvaging" the reputation of her ex-husband Charles Saatchi, "and destroying mine".
Of the millionaire art collector, she added, "If I did not go back to him and clear his name, he said he would destroy me."
Nigella was greeted by a sea of photographers and television crews as she arrived at court on Wednesday morning.
Wearing a long dark coat, she was flanked by a large group of police officers as she prepared to give evidence in the case against the Grillo sisters.
On Friday, jurors listened intently as her ex-husband Charles gave evidence. Details about the breakdown of the couple's 10-year marriage took centre stage over the allegations against the defendants.
Charles and Nigella before their split
The multi-millionaire said it was a "terrible, terrible mistake" that a private email he sent to Nigella, referring to her as "Higella" and claiming she had been "off her head" on drugs, had been made public.
"I'm utterly bereft that this private email to Nigella has come back to haunt us both," he said.
Charles said he had "no evidence" that the star had taken any drugs while they were married, and said that the allegations in his letter were made because he "didn't like reading what the Grillos said was the culture in my house".
The 70-year-old added, "I adore Nigella now. I absolutely adore Nigella and I'm broken-hearted to have lost her. I wanted her to be happy."