Constantinescu was said to be born in Chișinău, Bessarabia, at a time when the region was experiencing the aftermath of the October Revolution. (During the same month, the Moldavian Democratic Republic was proclaimed, leading to the union of Bessarabia with the Kingdom of Romania). According to fellow communist Alexandru Bârlădeanu, Constantinescu was born in Odessa. Widely believed to be an illegitimate son of the geologist Gheorghe Munteanu-Murgoci, Constantinescu retreated to a Romanian Orthodox monastery a short while after receiving his bachelor's degree.