Yyacheslav Gulevich, 57, headed up the Kemerovo organisation which dealt in drugs, weapons and human trafficking, as well as prostitution in Estonia.
He was arrested on August 2nd in the Spanish seaside resort of Mijas where he lived.
On the same day, three other group leaders were also detained in the Estonian capital Tallinn, where Kemerovo was was based, police said in a statement.
Estonian authorities had for two years been investigating the gang, which they said had been gripped by a fierce internal turf war which saw the previous boss Nikolai Tarankov shot in the head in September.
Gulevich was Tarankov's right-hand man and took over the organisation when he died, but feared for his life and fled to Spain.
In his home in Mijas, police found "an axe and a large knife hidden under his pillow," police said.
His fears appear to have been justified, as a member of the organisation was arrested in June in Catalonia in northeastern Spain, and "it appears he was in Spain to find Gulevich and murder him," the Guardia Civil police force said.
They added that among the three others arrested in Tallinn was a martial arts expert who recruited Kemerovo "soldiers" in his gym.