Britain has expressed regret to Saudi Arabia after what it described as a "disturbance" during a visit by an aide to Saudi Deputy Crown Prince Mohammed bin Salman to London last week.
Activists said on social media that a demonstrator tried to perform a "citizen's arrest" against Major General Ahmed al-Asseri as he tried to enter an office in London on Thursday.
Asseri is also the spokesman of the Saudi-led coalition engaged in fighting against Iran-aligned Houthis in Yemen.
Activists accuse Saudi Arabia of committing war crimes against civilians in Yemen, a charge Riyadh denies.
Britain's Foreign Office said in a statement that Foreign Secretary Boris Johnson had spoken about the incident with Prince Mohammed by telephone on Saturday.
"The Foreign Secretary expressed his regret at the disturbance to Saudi Major General Asseri's visit to London on Thursday,” the Foreign Office said.
Saudi state news agency SPA had earlier reported the telephone call and quoted Johnson as saying that the incident was under investigation.
The agency quoted the Saudi embassy in London as saying in comments on its Twitter account that Asseri was "safe" after some demonstrators tried to attack the general to obstruct his participation in a symposium by the European Council on Foreign Relations.
It said Asseri's schedule was not interrupted by the incident.
Saudi Arabia denies its forces in Yemen target civilians.
It says it takes extra care to avoid civilian casualties and accuses the Houthis of operating behind the cover of civilian targets.
The United Nations says the war in Yemen has killed more than 10,000 people, half of them civilians and caused a humanitarian crisis in the country.