RFE/RL's Tajik Service reports that all three of the participants who were arrested at the protest on November 4 were sentenced within hours by a court in Dushanbe to 15 days in jail.
Participants had gathered in front of the Justice Ministry to demand that it rescind its decision to recognize Masud Sobirov as the official leader of the Democratic Party.
The party's chairman, Mahmadruzi Iskandarov, is serving a 23-year jail sentence on terror-related charges that he denies.
Tajik voters go to the polls on November 6 to elect a president in a race that few see as a serious challenge to incumbent President Imomali Rakhmonov. Rakhmonov is seeking his third seven-year term.
Several major parties have boycotted or otherwise denounced the election, and critics have accused Rakhmonov's challengers of entering the field in doomed efforts aimed at ensuring authorities can point to a competitive election.
A media monitoring group has meanwhile warned in a prepared statement that steps taken by Tajik authorities against independent media have compromised the fairness of the upcoming election.
The Committee To Protect Journalists (CPJ) said state-controlled television has focused on the campaign of President Rakhmonov while virtually ignoring the four candidates opposing him.
The CPJ's Europe and Central Asia Program coordinator, Nina Ognianova, told an RFE/RL correspondent that Tajik voters face insurmountable official obstacles to informing themselves ahead of the vote.
"CPJ is very concerned that the Tajik authorities have deprived their citizens of independent sources of information just at the time when this independent information is most needed," Ognianova said.
CPJ also noted that at least four opposition and independent newspapers have been shut down in Tajikistan in the past two years. It also noted that the Communications Ministry in October blocked domestic access to five popular news websites that are published abroad and provide independent news and views.
(RFE/RL Tajik Service, Avesta, AP)