Beesly joined Congreve and Harrison, both now in London, in supporting the struggle of the workers in the building trades for shorter hours. He also attacked the economic theories used by critics of the "new model" trade unions of the 1860s. The notoriety he gained culminated in 1867, when he declared in the aftermath of the "Sheffield outrages" that a trade union murder was no worse than any other: he almost lost his post at University Hall and Punch dubbed him Dr Beastly. His radical agenda included promoting international solidarity among working-class leaders. He helped organise the most important pro-Union demonstration in England during the American Civil War, and he chaired the historic meeting (28 September 1864) advocating co-operation between English and French workers in support of Polish nationalism, which led to the formation of the International Working Men's Association (the First International), soon dominated by his friend Karl Marx.