london

Hmm, I'm minded to think this may well be earlier - the large scale works to reconstruct the Circle/H&C/Met platforms here was completed in c1936/8 and I suspect this may show that work. Also - trams not trolleybuses as you'd have found post-war. I'll check.

I agree - the buses suggest mid to late 30s, and tram routes round Kings Cross were converted to trolleybus operation in 1938.

Getty Images have a different photo of a tricolour protest in Trafalgar Square, dated 1922 The protest was for the release of Irish prisoners www.gettyimages.ae/detail/news-photo/flag-bearers-hold-th...

Streetview

If Feb 1922 (though fashion may be more recent) . . . ."a monster demonstration for the purpose of demanding the release of all Irish political prisioners at present in English jails including the Connaught Rangers was held on Sunday under the auspices of the Irish Self-Determination League of Great Britain. speeches were delivered from three platforms and the Tricolour was displayed around the monument . . . . Art O'Brien . . . . Sean Wickham . . . ." (Weekly Freeman Sat. 18 Feb 1922 Pg. 5)

[Aside] I never knew this: "The column also had a symbolic importance to Adolf Hitler. If Hitler's plan to invade Britain, Operation Sea Lion, had been successful, he planned to move it to Berlin." - en.wikipedia.org/wiki/Nelson%27s_Column

From Trove: A large Irish demonstration in Trafalgar-square demanded the release of 180 Irish prisoners who ara still in gaol, on the ground that their retention was at variance with the spirit of the Irish agreement. Article from The Advocate, Melbourne, 16 Feb 1922, in a longer article about Irish News. Doesn't give the date of the protest.

There was a protest on October 17th 1909 which fits the bill. Fermanagh Herald - Saturday 23 October 1909:

A demonstration organised by the British Social Democratic Federation, in co-operation with the Labour Party, Fabian Society, and other organisations, was held in Trafalgar Square, London, on Sunday afternoon, to protest against the execution of Senor Ferrer, and was attended by between nine thousand and ten thousand people.

John Spooner That's some crowd. Will the newspapers be our friend in uncovering Francisco Ferrer's alleged wrongdoing?

Date fairly securely established as 17 October 1909 then, thank you, and the postcard was sent on 10 September 1910.

Methinks the protest was a bit late.

en.wikipedia.org/wiki/Francisco_Ferrer

Francisco Ferrer was a Spanish radical freethinker, anarchist, and educationist behind a network of secular, private, libertarian schools in and around Barcelona. His execution, following a revolt in Barcelona, propelled Ferrer into martyrdom and grew an international movement of radicals and libertarians, who established schools in his model and promoted his schooling approach.

en.wikipedia.org/wiki/Francisco_Ferrer

The protest in Trafalgar Square was mild in comparison with what went on in Italy. It was "in the throes of a general strike" in protest against the "inquisitorial Spanish government", the Spanish consuls in Milan, Carrara and Florence resigned, and "immense bodies of soldiery" were guarding the Vatican day and night. (Daily Chronicle).,

It seems that the 'secular' aspect of Ferrer's teachings were seen as a threat to certain parts of the Spanish establishment.