The fighting between the presidential guards and army units supporting Jean-Pierre Bemba began after election officials announced that a second round run-off would be needed to decide who would be the next president.
A UN source said after a meeting between the rival military units: "They have both agreed to retreat to original positions and then continue talks. We are waiting to see if they take it seriously or not."
UN peacekeepers have carried out joint patrols with the presidential guard and Bemba's followers, telling soldiers from both sides to return home, another source said.
Kinshasa's main boulevard was sealed off by the peacekeepers and only diplomatic and military vehicles were allowed through.
The ceasefire followed a failed attempt to stop the fighting on Monday.
The 1,000-strong European Union rapid reaction force, which was sent to DR Congo to support UN peacekeepers during the elections on July 30, was reinforced earlier on Tuesday.
Sixty French, Portuguese and Swedish special forces soldiers and three helicopters were flown in from Gabon during the night follwed by a German-Dutch battalion of 200 men.
The UN's biggest peacekeeping force - more than 17,000-strong - is already deployed in Congo.
Presidential guards clashed with
supporters of Jean-Pierre Bemba
Bemba is under UN protection and their military vehicles have taken up position outside his house after foreign ambassadors had to be rescued from there on Monday when they were trapped by the fighting.
President Joseph Kabila had earlier said that all Congolese troops in the capital should return to barracks immediately, a spokesman said.
"The only thing that is important at the moment is that the situation in Kinshasa returns to normal," Kabila said in comments relayed by his spokesman.
The fighters are part of rival army factions that supported either Bemba or Kabila during the 1996-2002 war in the Democratic Republic of Congo. The fighters entered the army after the war ended but remained loyal to their leaders and stayed largely out of the formal chain of military command.
Kabila gained 44.81% of the votes in the first round of the election, short of the 50% needed to win the presidency outright but well ahead of Bemba who had 20%.
The deciding run-off vote is planned for October 29.