In Bucharest he met several trade unionist active in the waiter's guild, such as Gheorghe Niculescu-Mizil, Iancu Olteanu, and Marian Cristescu, who introduced him to socialist ideas.
As Romania joined the war on the side of the Entente in 1916, Leonte Filipescu was drafted and sent to the front, but was soon captured by the German Army and interned in a concentration camp.
He succeeded in escaping along with several prisoners, and by the spring of 1917 he was back in Bucharest, taking part in the demonstration organised by the Socialists in front of the German Army Headquarters in occupied Romania.
In the first post-war years, Leonte Filipescu became one of the leaders of the left-wing grouping of the Socialist Party of Romania, and participated in the organisation of several strikes and mass demonstrations in Bucharest.
Following the authorities crackdown on the 1920 Romanian general strike, which he helped organise, he went into hiding, while continuing to spread pro-socialist manifestos and pamphlets.