After the ideological split of the BWSDP, Kolarov cast his lot with Dimitar Blagoev's Tesniak (Narrow) wing of the party, which espoused revolutionary socialism.

[1] In April 1919 the Bulgarian Communist Party was established, with Kolarov elected the first Secretary of its Central Committee.

[1] Kolarov was elected to the Executive Committee of the Communist International (ECCI) and became a key functionary in the Comintern.

Early in 1923, he travelled to Central and Western Europe on behalf of the Comintern, attending Communist gatherings in Paris, Frankfurt, Oslo, and Prague.

[3] 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.

[3] 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 remained a top official of the Communist International, presiding over the body's debates at the 5th World Congress of 1924.