Francesca Grillo said Mr Saatchi had a “personal vendetta” against her and sister Elisabetta, who also faces a charge of fraud.
The sisters are alleged to have spent £685,000 (€809,826) on credit cards belonging to the celebrity couple, who were divorced earlier this year.
Francesca, 35, told Isleworth Crown Court that Mr Saatchi asked to see her at his home in July after his financial director Rahul Gajjar confronted her and Elisabetta, 41, with credit card statements. The court heard Mr Gajjar had asked the sisters to sign a letter the previous day, admitting dishonesty and promising to work for Mr Saatchi and Ms Lawson for a reduced salary for an unlimited period of time.
The Grillos had not signed the letter and asked for a copy of it while they considered their options, the jury heard.
After she took a cab to Mr Saatchi’s house, Francesca said he told her she was “stupid” and asked her: “Why didn’t you listen to Rahul?”
She said she was accused by her then-boss of buying a house on the credit card, which she denied.
Francesca, of Italian descent, told the court: “He was banging on the table . . . He said I would end up in handcuffs.”
She said the situation became “quite scary” as Mr Saatchi told her: “Hide anywhere in Italy but I will find you and destroy you.”
She added: “He said he was going to destroy me and hunt me down. That was his words.
“His voice was shouting and he was banging on the table and accusing me of various things that were not true.
“The more he got upset, the more I got frightened.
“You don’t cross Charles Saatchi, everyone knows that.”
Specifying what had upset her, she added: “The way he accused me, the things he said about me and the fact he went on a personal vendetta.”
Francesca and her sister spent three hours locked in a cell after their arrest, she told the jury.
The Grillos wrote a letter to Ms Lawson saying they were “reaching out” to her and asking for forgiveness, the jury was told. “We never meant to be seen to be disloyal or seen like we took advantage of our positions,” the court heard they had written.
In the letter, the sisters said Ms Lawson and Mr Saatchi were their “English family”, adding: “We saw you like mother and father figures.”
She told the court, with emotion making her voice crack: “They were my family.”
Grillo left in tears, telling Judge Robin Johnson she needed five minutes to compose herself. The trial continues.