Thousands of opposition supporters have been demonstrating in the Democratic Republic of Congo's capital, Kinshasa.
Tshisekedi supporters say they still have not registered to vote
Followers of veteran opposition leader Etienne Tshisekedi are demanding a repeat of voter registration for elections scheduled in June.
Visiting UN head Kofi Annan has said the polls pose "major logistical challenges, if not nightmares".
The first free elections in 45 years are to end a power-sharing period after a five-year civil war.
During the peaceful march to the UN headquarters, banners held up said: "Tshisekedi or Iraq" - an apparent warning of fears that conflict could reignite if Mr Tshisekedi's demands are not met for more time to register voters.
His party, the UDPS, also want a role in helping to organise the elections.
"The authorities want to reject the person who is most popular. The people are not happy," a Tshisekedi supporter told the BBC's Arnaud Zajtman in Kinshasa.
Mr Annan has urged Congolese President Joseph Kabila and others to make the voting as free and fair as possible.
The present United Nations force of nearly 17,000 troops is the UN's largest.
The UN secretary-general says a planned deployment of European Union troops is also expected to act as a rapid reaction force during the polls.
More than 1,000 people are estimated to be dying each day in the country, where fighting - particularly in the east - continues.
Hostilities between rival militias and government forces continue to force hundreds of thousands of displaced people from their homes.