A Russian man was jailed for four years by a British court on Friday after the U.S. president's son Barron Trump had witnessed an assault on a woman friend of his during a video call and alerted police.

The 20-year-old youngest son of President Donald Trump told police that he had called the victim at her home in London in January last year from the U.S. and saw she was being attacked.

Officers arrested Matvei Rumiantsev, 22, who had been dating the woman on and off for several months.

"At one stage in the violence, there was a call to or from her friend Barron Trump," Judge Joel Bennathan told Snaresbrook Crown Court as he handed down the sentence.

"He saw you beating her up and you held the phone and filmed her, an angry act to humiliate her. Mr Trump properly and responsibly, despite being in the United States, made sure the emergency services here were called and he told them what he had seen."

As a result of the attack, the woman, who had also made emergency calls to police during the attack, suffered bruising to her chin, arm, and wrist, as well as scratches while a clump of hair fell out.

Rumiantsev was found guilty in January of assault and perverting the course of justice but acquitted of rape and intentional strangulation, and given a two-year prison sentence for both offenses with the jail terms to run consecutively.