Metropolitan Police have arrested 20 people on suspicion of belonging to a banned Islamic-extremist group.
The group of protesters were seized outside the U.S. Embassy in London on suspicion of their membership in the banned group Muslims Against Crusades (MAC).
According to Metropolitan Police, there were 22 arrests in total: 20 on suspicion of belonging to a proscribed group, one for obstruction and another for violent disorder.
Those arrested are currently being held in central London police stations.
Police said that a small number of people are still at the demonstration, thought to be against the American military's use of drones.
In a series of Tweets, former MAC leader and hardline Islamist Anjem Choudhury described the afternoon's events.
"Police have kettled in & arrested all the Muslims who were demonstrating outside the US embassy today against the murderous US drone strikes," he wrote.
"They used aggressive tactics beating Muslims and treating them heavy handedly including young boys with no reasons given or rights explained.
"This latest assault is evidence of the brutal apartheid reality in which Muslims suffer under the UK Cameron. Among them is Sh. Abu Izzadeen."
Muslims Against Crusades was banned by Home Secretary Theresa May on 10 November after it was announced that the group planned to demonstrate at Remembrance Day celebrations.
Several members of the group were previously hauled before magistrates for burning poppies at military parades, resulting in one of them being fined £50 for a public order offence.