Faure Gnassingbe was backed by the military as president following the death of his father, Gnassingbe Eyadema.
However, Yawovi Agboyibo - head of the Action Committee for Renewal - said on Thursday the rally protest would begin in the coastal capital Lome early on Saturday morning.
He invited citizens from around the country to join a march co-sponsored by the six main opposition parties in the west African state.
The six parties had earlier called a two-day strike, urging people to stay home in protest at the swearing-in on Monday of Gnassingbe over a chorus of international protest.
But few people heeded the call, cowed by the 38 years of repressive rule by Eyadema, West Africa's strongman, and the tight grip maintained on the population by the military.
Saturday's rally will defy a ban imposed on Monday against all public demonstrations by the government in the guise of national observations of mourning for Eyadema, who died en route abroad in search of emergency medical treatment.
Authorities have since ramped up their warnings against those who wish to cause trouble, threatening an "energetic reaction" by security and defence forces.
But Agboyibo waved off such threats, noting that "we are right now in a context where attempts to curb public freedoms are not to be adhered to."