Activist attempts to arrest Saudi general for 'Yemen crimes'
Sam Walton confronted Major General Ahmad al-Asiri as he arrived for a meeting, however was quickly ushered away by Asiri's bodyguards.
One protester smashed an egg on the military spokesman's back, while others were prevented from blocking his car by police who were at the scene.
Footage posted to social media shows that the general then turned around after entering the building and gestured his middle finger to the protesters.
"Asiri represents a regime that has killed thousands in Yemen and shown a total contempt for international law. I tried to arrest him because of the war crimes he has overseen and propagated for, but he was surrounded by bodyguards who roughly forced me away," said Walton.
"Asiri shouldn't be welcomed and treated like a dignitary, he should be arrested and investigated for war crimes."
Asiri, who is in London for a think tank discussion on Yemen's civil war, is the Arab military coalition's main spokesman and has often made controversial and inaccurate statements about his country's military intervention.
Last year, he denied that the Saudi army had been using British-made cluster bombs in Yemen. This claim about the deadly munitions, which are banned from use by an international convention, was later revealed to be untrue.
The Saudi-led coalition's war against Houthi rebels in Yemen has cost more than 10,000 lives since March 2015 and brought the country to the brink of famine.
According to the UN, the fighting has displaced more than three million people, and more than two thirds of Yemen's population of around 18.8 million people need aid.
Some 7.3 million people are estimated to be close to starvation and 462,000 children suffer from serious malnutrition. Without $2.1 billion in international aid, the UN warns that Yemen will suffer a famine in 2017.