The PCI was founded by split from the socialists at Livorno
Evidence
The first communist to speak, Secondino Tranquilli (subsequently known as Ignazio Silone) the editor of the youth paper, asked the communist delegates to “burn the effigy of unity”. They left the conference singing the Internationale , and held their own founding congress in a neighbouring hall. — Antonio Gramsci, Selections from the Prison Notebooks, 111