Proshian, Prosh was born in 1883 in Ashtarak in Armenia. Son of an Armenian writer.
As a student in Odessa, joined the SR Party. After the 1905 Revolution, exiled to Siberia. Escaped from East Siberia.
In 1913, arrested and sent back to Siberia. Escaped abroad. Returned to Russia after the February Revolution 1917. Left-wing SR, main proponent of a coalition with the Bolsheviks, twice expelled from the SR Party.
Member of the Central Committee of the left SRs. In the first Soviet (coalition government), Minister for Mail and Telegraph. In March 1918, together with other left SRs, left the coalition in protest against the Brest peace treaty.
One of the leaders of the left SR revolt in Moscow in the summer of 1918. After their defeat, went underground.