This remarkable photograph was published in The Illustrated London News on 15 September 1934 and shows the Fascist demonstration in Hyde Park of 9 September. There was an "anti-Fascist counter-demonstration" at the same time, same park–the two sides were divided by the "No Man's Land" path in the middle, screened by police on each side. The crowd at the left/middle is the fascist group–easily discriminated by their salute and then their visual sameness, so many of them wearing the signature black shirts. At bottom/right/top is the counter-demonstration group, which is far larger–they were orderly but not having any patience for Hitlerism.
Which is a detail from:
I found a handbill for one of the opposing groups at the demonstration: the Young Communist League, which evidently showed up in force. In the caption of the above photo there is no mention of the party affiliation of the anti-fascists, except to quote witness Will Rogers saying "the Blackshirts were holding one meeting. Two hundred yards away the Communists were holding theirs. And in between was all of London lauhing at the both of them". According to a quick search I'm not sure that there were this many communists in all of London in 1934–I assume the anti- crowd was very mixed.
(These Blackshirts should not be confused with Albanian/Indian/Italian blackshirts, or German brownshirts (brown maybe because black was traditionally used for Christian Democrats?), or American silvershirts, though some do bear some resemblence. In the other color-shirt-political-affiliation categories there are, for example, the redshirts of Italy, the blue- and greenshirts of Ireland, the goldshirts of Mexico, the greyshirts of South Africa, the greeshirts of Romania, and the blue shirts of Taiwan).
Source: British National Archive, here.
The "Mosley" here Sir Oswald Mosley (1896-1980), founder of The British Union of Fascists in 1932 which in 1936 changed its name to the British Union of Fascists and National Socialists and then in 1937, slimming it down to the British Union, until it was disappeared by the government in 1940 in a 'defence of the realm action" under Defence Regulation 18B.
Mosley and his wife were arrested in 1940 and spent a few years in relatively high privilege in prison, a situation granted by Winston Churchill. They lived in their own inner-prison cottage, with a garden and servents. They were released in great controversy in 1943 and seem to have spent decades in the far right spectrum publishing and promoting questionable and of course distasteful political viewpoints.