After al-Dosari and his friend left the café, two men started following them.
“After we walked 100m or so we were approached by two guys from behind,” al-Dosari told The Independent on Sunday.
“They started shouting at me... they were saying ‘who are you to talk about the family of al-Saud?’ I think they knew where I was from Snapchat, they recognized me easily.”
Viral footage shows a man, wearing jeans and a light shirt, punching al-Dosari in the face as terrified shoppers and families flee the scene while others attempt to separate them.
Another man, wearing a grey suit and a wired earpiece, then follows the activist down the road before being dragged backwards and restrained.
His friend Alan Bender, a Canadian businessman, said the two men accused al-Dosari of being a “slave of Qatar” – a foe of Saudi Arabia – and threatened to “teach him a lesson”.
“I told them this was not Riyadh, this is London, and the guy immediately said: ‘F*** London, their Queen is our slave and their police are our dogs,” he added.
According to Bender, the pair shouted “how dare you curse Prince Salman, we won’t allow it” and insulted al-Dosari, his mother, sisters and family in bad language.
Bender added that the men only fled when some people shouted that the police were coming.
The attack left al-Dosari bleeding from the mouth and he received treatment at the scene in an ambulance.
He was later transferred to Notting Hill Police station, where he reported the attack although he believed that both men have since returned to the kingdom.
The activist vowed that he will not retreat and will continue his human rights work, which is well-known in his home country because of his sizable social media following.
“They were trying to intimidate me, they were trying to scare me, but I will not stop,” said al-Dosari.
“I had never thought they would attack me here – anywhere else in the world yes – but in the UK and in front of Harrods in broad daylight? This is the area where I feel most safe.”
He also denounced the UK government for being too “friendly” with the Saudis in spite of the kingdom’s human rights record.
Al-Dosari, who has been living in the UK since he fled Saudi Arabia in 2003 due to his political views, said, “I’ve never been back.”
“It’s not safe for me there, it’s not safe for anybody who tweets their opinion.”
Al-Dosari’s YouTube videos target the Saudi regime. He has nicknamed Crown Prince Mohammad bin Salman “the tubby teddy bear”.
Bender noted that his friend had received threats before and the attack did not surprise him.
“I am worried about his safety,” he added. “The message [from this attack] says ‘no one can stop us’.”
According to a spokesperson for the Metropolitan Police, officers were called shortly before 6.15 pm on 13 August to an assault on Brompton Road, SW1.
“At the scene the victim, a man aged in his 30s, had suffered bruising to the face. He did not require hospital treatment,” they added.
“Officers from Kensington and Chelsea are investigating. The suspects are believed to be two males aged in their 30s. At this stage there has been no arrests. Enquiries continue.”
Last month, British Foreign Secretary Jeremy Hunt defended close ties with Riyadh, telling the BBC that the UK's relationship with Saudi Arabia stops bombs going off on the streets of Britain.
Hunt also said he was "deeply shocked" that at least 40 children were killed in northern Yemen when a Saudi airstrike hit their school bus.
Both the UK and US supply Saudi Arabia with munitions that are used in Yemen. An American laser-guided bomb is believed to have been used in a coalition strike in early August which killed more than 50 people.