The blaze came after three people were killed on Wednesday in clashes with police on the sidelines of an opposition rally in eastern DR Congo.
The fire broke out around 2am in a building where election materials were being kept, the head of the Independent National Election Commission (CENI) said.
A thick cloud of black smoke was still visible above the city by early morning, AFP reporters said.
The cause of the blaze was not immediately clear but the run-up to the vote has seen a wave of violence.
On Wednesday, clashes erupted in Kalemie, a town on Lake Tanganyika, as opposition candidate Martin Fayulu was campaigning there.
Two witnesses said live rounds were fired after the opposition candidate arrived and headed to the rally venue.
Fayulu blamed the violence on police as well as on "armed youths on drugs" who, he said, were "dressed in PPRD clothing," a reference to the country's ruling party.
On Tuesday, two of Fayulu's supporters were killed and 43 hurt in clashes at a rally in Lubumbashi, the Democratic Republic of Congo's second-largest city.
Fayulu, 62, a little-known lawmaker and former oil executive, has made a late surge after being named the joint candidate for several opposition parties.
The DRC is in the throes of a major campaign ahead of the December 23 election to choose a successor to President Joseph Kabila, who has ruled the vast central African country since 2001.
The nation has never known a peaceful transition of power since it gained independence from Belgium in 1960.
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