Gwyneth Paltrow opened up about the "traumatic experience" she suffered at the hands of an alleged stalker as she testified against him in court on Monday (08Feb16).

The Avengers star claims Dante Soiu has been harassing her for 17 years, but he was found not guilty by reason of insanity in 2000, after allegedly bombarding Paltrow with sexually explicit messages and unwanted gifts.

He spent a number of years in a mental institution and was taken into custody again last year (15) after reaching out to the mother-of-two once more.

Soiu currently stands accused of felony stalking charges after reportedly filling up her mailbox with 66 letters between 2009 and 2015.

Paltrow came face to face with the suspect in Los Angeles Superior Court on Monday, when she was called to describe how she felt about his unsolicited correspondence on the first day of his new trial.

During her hour-long testimony, the 43-year-old admitted she had been left upset by many of his letters, in which he discussed his intentions of marrying the actress, while she also feared for her safety as others contained religious references and statements detailing her death.

In one such note, Paltrow told the judge Soiu, 66, had threatened to use a scalpel "to cut out sin" in her body, while another described her "bowing to death", reports The Associated Press.

"I felt very upset by it," Paltrow said. "This has been a very long and very traumatic experience already."

Soiu's attorney, Lynda Westlund, admits her client did send letters to Paltrow, but insists he is a Christian who never included anything explicit in the messages, which included forwarded notes he had also sent to President Barack Obama and fellow actress Angelina Jolie, who is now married to Paltrow's ex-fiance, Brad Pitt.

Soiu, from Columbus, Ohio, has pleaded not guilty to the new charges.

The court appearance took place a day after Paltrow attended the Super Bowl in Santa Clara, California with her kids Apple and Moses. They were there to cheer on Gwyneth's rocker ex Chris Martin, the children's father, and his band Coldplay as they performed with Beyonce and Bruno Mars during the half-time show spectacular.

Paltrow and Martin split in 2014 after 10 years of marriage and are currently in the midst of a divorce.