Masud Sobirov said 60 out of the 68 HDT delegates who attended the August 27 party congress elected him as their new chairman.
But those party members who remain loyal to jailed HDT leader Mahmadruzi Iskandarov say the election and congress were illegitimate.
They accuse Sobirov, who heads the HDT's Vatan (Motherland) faction, of working secretly with the government to undermine the opposition party ahead of the November presidential election.
Iskandarov is serving a 23-year prison term on terror-related charges he denies. The exiled HDT says he was abducted and forcibly returned to Tajikistan from his Russian exile last year.
(with material from Avesta, Asia Plus)