Presidential polls will be held in Togo on 24 April, election officials say, after a month of turmoil following the death of President Gnassingbe Eyadema.
The opposition called for Mr Faure to stand down
Opposition parties have agreed to contest the poll despite rules which bar their main candidate from standing.
However, Gilchrist Olympio's party says more time is needed to organise a "transparent and honest" election.
The succession of Mr Eyadema's son, Faure Gnassingbe, to the presidency was widely condemned as a coup.
After West Africa imposed sanctions, Mr Faure stepped down, agreeing to polls within 60 days as stipulated in the constitution.
Mr Faure has said he intends to contest the polls as the candidate of the ruling party.
"It's surrealistic. I don't think it's possible to organise a transparent and honest election in that period," said Jean Pierre Fabre, secretary general of Mr Olympio's Union of Forces for Change (UFC).
"We are ready to participate in the election, but we are not we are not suicidal, we are going to remain vigilant," he added.
But under the constitution, candidates need to have lived in Togo for 12 months to stand as a presidential candidate, which bars Mr Olympio from standing.
Since a 1992 assassination attempt, Mr Olympio has lived in exile in Paris and was barred from taking part in the 2003 presidential elections because he did not live in Togo.
Leopold Messan Gnininvi, the spokesman for the six-party opposition alliance, described as regrettable the disqualification of Mr Olympio.
"We, his colleagues in the struggle for the restoration of democracy in Togo, will continue to show solidarity with Mr Olympio until we find a way of removing that prohibitive clause from the statute books," he said.
On Wednesday, Mr Olympio told the BBC he would give his support to whoever his party nominated.
"My 40-year struggle for the restoration of democracy in Togo is not a personal one.
"I will give my support to a candidate to be nominated by the party," he told BBC Afrique.
Mr Faure - who was president for three weeks following his father's death - will be standing as the ruling party's presidential candidate.
After Mr Faure stood down, Ecowas lifted the sanctions it had imposed on Togo.
Angry protesters had taken to the streets following the appointment of Abass Bonfoh as interim president last Friday after Mr Faure's resignation.
They demanded the former parliamentary speaker Fambare Ouattara Natchaba take over as interim president, in accordance with constitutional procedures.
Opposition parties are now expected to submit the names of their three representatives to serve on the newly created National Independent Electoral Commission.