Tension is high in the DR Congo ahead of a presidential run-off pitting incumbent Joseph Kabila against ex-rebel leader Jean-Pierre Bemba.
Joseph Kabila (left) is running off against Jean-Pierre Bemba (right)
Mr Kabila won first round polls on 30 July, but fell just short of the 50% needed for outright victory.
Armed clashes between the two men's security forces in Kinshasa after results were announced left at least 23 people dead.
A UN official in Kinshasa described the situation as "tense but manageable".
Brigadier General Carl Modey, who commands UN peacekeepers in Kinshasa, said extra troops were on standby in the east of the country.
In the capital, the UN is being backed up by a special European Union military force.
Mr Kabila, meanwhile, said he would crack down on any "provocation".
"The situation is grave," he said in a TV interview on Friday. "All measures have been prepared."
The BBC's Mark Doyle, in Kinshasa, says that while the former warlords standing in this election have made promises to behave like democrats, the foreign soldiers are not here for nothing.
Sunday's run-off, which will conclude the country's first fully democratic polls since independence in 1960, is aimed at restoring peace to the war-torn nation.
The UN peacekeeping mission in DR Congo is its largest worldwide
Warring factions signed a peace deal in 2002 after a five-year conflict in which four million people are estimated to have died.
But both presidential candidates still have armed forces loyal to them and there are fears of further clashes after the result.
There has been some violence in the election run-up. Four people were killed on Thursday in clashes between rival factions in the north of the country.
On Friday, UN forces used an armoured vehicle to free one of Mr Kabila's key allies from a radio station owned by Mr Bemba after an armed stand-off.
Both candidates have promised to urge their supporters against violence and abide by election rules.
William Swing, the head of the UN mission in DR Congo, said the vote was the most important vote in Africa since the 1994 poll that ended apartheid in South Africa, Reuters news agency reported.
"These are the biggest elections that the United Nations has ever supported," he said.