[1] During this time, Domaševičius participated in an illegal student circle and became acquainted with the literature of the Russian Narodniks movement.

After his studies in Kyiv, Domaševičius worked in St. Petersburg with a famous Russian gynecologist professor in a medical clinic.

[5] After returning to Lithuania, Domaševičius was invited by Alfonsas Moravskis, whom he knew since his days in Panevėžys, to participate in political and social activities.

Domaševičius acquired the pseudonym Teodoras and read various lectures in secret meetings, as well as organized educational, economic and political workers' unions.

By the initiative of the Social Democrats, so-called struggle funds were created to support the strikers, and trade unions were established.

In 1897, Domaševičius was arrested and jailed in Vilnius[4][2] for his political activity for a month and a half, but was released due to insufficient evidence.

[3] He lived in Omsk, as well as Karkaraly and Semepalatinsk, in which he freely learned Italian, English, Hebrew, French, German, Polish as well as Lithuanian languages.

[1][5][4] In 1910, he established a private clinic and a hospital specializing in gynecology, in which poor women were treated free of charge.

Domaševičius also wrote articles for the magazines Medicina ir gamta (Medicine and Nature) and Darbo balsas (The Voice of Work).

After the October Revolution in Russia in 1917, Domaševičius joined the Social Democratic movement again, and also was selected as the chairman of workers' council of the Naujoji Vilnia organization.

In 1919, he founded his own Lithuanian Communist Party,[2][1] made up of parts of the Social Democratic and Naujoji Vilnia movements, becoming its chairman.

[2][4][1] A new department of obstetrics and women's diseases was established at the Saint James hospital in Vilnius under the efforts of Domaševičius.