She said it was the first time she stood up to him and she believed Depp was about to push her sister Whitney down the stairs.
The actress said she had been "so in love" with Mr Depp and had "always held out hope" he would get clean.
She claimed blood was pooled on the floor and what looked like dripping blood, "I didn't know what to make of it" she told the court.
She was then questioned by Johnny's lawyers about why she'd never brought up the allegation involving Kate before.
Heard said the "information" was "fresh" in her mind at the time of the alleged incident and that she reacted "in a flash" to protect her sister. She is the only witness in the case, apart from Heard, to offer such a first-hand account.
Amber Heard, testifying for a third day in ex-husband Johnny Depp's libel suit against a British publisher, acknowledged breaking a bottle during an argument in Australia-but said he responded by chucking dozens of bottles at her like they were "grenades or bombs".
Ms Laws also asked Ms Heard about her first allegation of violence against Mr Depp, which she says happened after she remarked on his "Wino Forever" tattoo.
Laws made much of a series of video clips from an elevator at the couple's Los Angeles apartment building that she claimed showed no evidence of any injuries on Heard's face after an incident on May 21, 2016, which ended up with police arriving on the scene.
Asked why she had agreed to return with Depp to the same house in Australia a couple of months later, she said: "Johnny - the monster - almost killed me".
While in London's High Court this morning (July 22), Amber alleged that the actor, 57, was emotionally and physically abusive towards her.
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The court heard Ms Heard texted Mr Holmes just before 6pm to say: "Success - he's coming down".
The defamation case, now underway in a London courtroom between Depp and News Group Newspapers, pertains to an article in British tabloid The Sun that referred to him as a "wife beater" during his short-lived marriage to actress Heard, who accused Depp of being abusive in 2018.
Ms Heard told the court: "I was so in love with Johnny at that time". I retired further at the bar and he didn't stop. "I had bruises all over my body", she said, after Laws argued a nurse who had examined her shortly after the row said she found no bruises.
But Heard today denied faking photos of her alleged injuries after they were shown to the court.
Asked if she if she had benefited either in her career or financially as a result of the proceedings she said "what woman has ever benefited from being the victim of domestic violence".
But she also texted her mother saying she should not worry because Mr Depp was not being attacking her, "It's OK mum he's not being violent with me".
She told her mother that Mr Depp was having insane mood swings because he had spent more than a day drinking and taking drugs.
Asked why she told her mother that he had not hurt her, Ms Heard said that her father had been violent towards her mother throughout their marriage, and that she became concerned her mother would tell her father about Mr Depp's violence towards her.
"She said she feared for her life, and he grabbed her by the hair and dragged her so she had a chunk of hair missing on the crown". They married in Los Angeles in February 2015. Heard replied: "I didn't include every detail or every thought".
This week's evidence centred around the days before Heard filed for divorce.