"They were caught trying to change figures on a results sheet for the presidential vote," said Dieudonne Mirimo, spokesman for the independent electoral commission.
The six suspects have been taken to court.
19 of the 32 candidates in the presidential election have criticised "numerous acts of fraud" and called for a new vote.
The group of candidates said extra ballot papers and electoral cards had led to stuffed ballot boxes and accused observers of a "complicit silence."
They warned that the elections would not bring peace or a new political order to Congo.
None of the 19 are among the front-runners nearly two weeks after the election.
"Let them take their complaints and proof to the supreme court," Mirimo said.
Many international observers noted irregularities during the voting and counting, but none so far have been considered serious enough to affect the outcome.
Current president Joseph Kabila leads Jean-Pierre Bemba, a former rebel leader and current vice president, with 2.1 million of about 20 million votes counted from the poll on July 30.
Kabila has 48% of the vote compared to Bemba's 20%.
None of the candidates are expected to win an overall majority and a second round will probably have to be held between the top two.
Most of the voting papers have now been collected from the tens of thousands of polling stations across Congo, a country the size of Western Europe with few paved roads.
The results are expected to be announced on August 21.