In June 1923, Kolarov spoke at the 3rd Enlarged Plenum of ECCI in Moscow before returning secretly to Bulgaria at the end of the month. There he was promptly arrested but released on 5 August. Upon his release, Kolarov played a critical role along with Georgi Dimitrov in convincing the Bulgarian Communist Party to organize an insurrection in accordance with Comintern instructions. Kolarov was a member of the revolutionary committee which launched that uprising in September 1923. This attempted revolution failed and Kolarov was forced to flee to the Soviet Union by way of Yugoslavia and Austria. He remained in exile for more than two decades.