A policeman guarding presidential candidate Azarias Ruberwa was shot dead on Friday night as his entourage exchanged fire with the security personnel of President Joseph Kabila outside Kinshasa.
A UN spokesman said two other people were injured in the shooting that took place as Mr Kabila, the firm favourite to win the elections, was heading for a final rally before campaigning ended at midnight.
Mr Kabila told supporters Sunday’s landmark vote "will allow the country to emerge from the crises and uncertainty" that has gripped the former Belgian colony for more than four decades since independence in 1960.
He called for a "massive turnout" and promised to provide roads, running water and higher education throughout the impoverished central African country.
There are more than 30 presidential hopefuls but only former rebel leader
Jean-Pierre Bemba is seen as a strong rival to the 35-year-old Kabila, who took power in 2001 and enjoys the support of the West.
On Thursday night, Mr Bemba's last rally descended into bloodshed as his supporters went on the rampage, hurling stones and shooting at police and torching a church.
Three policemen and a civilian were killed.
In the troubled east of the country two people were shot dead in an ambush on government vehicles on Thursday, while on Wednesday an election candidate was forced to watch as gunmen killed his mother in the central mining town of Mbuji-Mayi.
The United Nations said it expected strife as 25.7 million Congolese go to the polls to elect a president and 500-seat parliament, but predicted the vote would be largely free and fair.
"We believe these will be good elections, but not perfect ones, there will be problems," UN envoy Ross Mountain said.
Mr Bemba issued a call for peaceful polling, echoed by the United Nations,
United States, European Union and South African President Thabo Mbeki, all of whom are funding the vote in the hope that it will help to stabilise central
Africa.
President Mbeki helped broker an accord to end a five-year war in the region that began when neighbouring Rwanda invaded the DRC in 1998 to topple Mr Kabila's father.
Dubbed "Africa's World War", it claimed nearly four million lives and ruined what was left of the former Zaire's fragile economy and infrastructure after 37 years of corrupt rule by former dictator Mobutu Seso Seko.
Observers have pointed to the huge logistical challenge of holding elections in the war-ravaged nation, where some ballots were this week transported by canoe to villages in the remote north.
But the biggest concern has been persistent ethnic fighting and looting of the DRC's vast mineral resources, which has hampered campaigning.
Ethnic Hutu politicians complained that it was impossible to reach voters in the Tutsi-dominated eastern provinces.
And Mr Kabila was forced to hold a rally in Mbuji-Mayi in the relative safety of the airport because the diamond town is a stronghold of opposition leader Etienne Thisekedi, who is boycotting the vote.
Peace prospects received a boost this week when the rebel Congolese
Revolutionary Movement agreed to a truce in eastern Ituri province where they have in recent months attacked army positions and kidnapped UN peacekeepers.
Sunday's elections will be policed by a thousand EU soldiers deployed in the DRC, with as many on standby in neighbouring Gabon to evacuate foreigners if violence flares.
The EU force is backing up the 17,600-strong UN peacekeeping mission in the DRC while 1,500 international observers will monitor voting.
Hopes for a strong turnout grew this week when the Catholic Church dropped its earlier refusal to endorse the elections.
But it warned that there will be no lasting peace unless the West helps to rebuild the DRC and local politicians learn to put the good of the people above clan loyalties and personal interest.
"Millions of people will have died in vain and millions more will face the same fate," Church leaders said in a statement.