Mosley planned to march thousands of his British Union of Fascists through the East End of London, an area where many Jews lived. This attracted a lot of opposition which prompted the police to provide a 10,000 strong escort for the march. Most of the marchers and also the counter-protesters were not locals, they came here specifically for the event.
The anti-fascists (Jews, Socialists, Anarchists, Communists, Irish) put up barricades and attacked the police when they tried to clear the street. People were throwing things at the police from the windows along the street. Mosley agreed that his marchers should go elsewhere, leaving the police to battle it out with the anti-fascists. 150 were arrested and about 100 people including women, children and police were injured. Two important outcomes: political parties were banned from wearing uniforms; it became obligatory to obtain police consent for political rallies.
Almost exactly 83 years later the similar anti-fascist Battle of Lewisham took place.