After bursting into tears, Amber Heard alleged that her ex-husband Johnny Depp hit her and that the first strike left an unforgettable mark in her life.

The Aquaman star said she had not known how to react following the alleged incident as she gave evidence in the U.S. defamation trial brought against her by her former partner.

Depp is suing Heard for libel over a 2018 article she wrote in The Washington Post, which his lawyers say falsely implies he physically and sexually abused her while they were together.

Heard told Fairfax County District Court in Virginia on Wednesday that the early days of their relationship were "a beautiful and strange time" and that she had "fallen head over heels in love" with Depp.

But she said that during an incident in 2013 the actor had slapped her three times after she had laughed about one of his tattoos.

"(It was) seemingly so stupid and insignificant I will never forget it, it changed my life," Heard said.

"He slapped me across the face, and I laughed because I didn't know what else to do.

"I thought this must be a joke ... I didn't know what was going on. I didn't want this to be the reality."

She continued: "I know you don't come back from that, I'm not dumb, you can't hit a woman, you can't hit a man, you can't hit anyone.

"I knew it was wrong and I knew that I had to leave him and that broke my heart because I didn't want to leave him ... the best thing that ever happened to me.

"I wish I could sit here and say I stood up and I walked out of that house and I drew a line and I stood up for myself.

"I was just looking at the dirty carpet trying to will myself to get up, to walk out of the door."

Heard said Depp had then started crying and got on his knees to apologize and told her he had "put the monster away."

"(He said) I've done it before, I'll do it again, it's done," she said.

"I sat in my car and I felt like I sat there forever ... just trying to will myself to have the strength to know what I should do at this moment because I was heartbroken."

Heard added that in the following days Depp had apologized "profusely," telling her "I'd rather cut my hand off than lay it on you."

"He had that way of talking like poetry," she said.

Asked if she had stayed in the relationship following the incident, she replied: "Yeah I did, I believed there was a line, he wouldn't cross it again and that was it."

Heard earlier told the court that the couple had engaged in a "whirlwind romance" following the press tour for their 2011 film "The Rum Diaries."

"We had to be really under the radar because as Johnny pointed out, the world would blame me and call me a home-wrecker even though I had nothing to do with it," she told the court.

"It was beautiful ... I felt like this man knew me and saw me in a way no one else did.

"When I was around Johnny I felt like I was the most beautiful person in the world. I felt seen."

Heard added that Depp had shown "lavish expressions of love" and that they had lived in "little bubbles of secrecy."

"It felt like a dream, it felt like absolute magic," she said.

"I fell head over heels in love with this man."

The court previously heard evidence from forensic psychologist Dr. Dawn Hughes, who said Heard had endured multiple acts of violence and sexual abuse at the hands of Depp.

Dr. Hughes said that the alleged acts of sexual violence, which she graphically detailed to the court, were often a result of the actor's "drug-fuelled rage."

The court has already listened to four days of evidence from Depp, who said he was "obsessed with the truth" and claimed that it was he that was the victim of domestic abuse. The trial continues.