Two men dressed in civilian clothes had shot dead Kisoni Kambale, a wealthy and well-known local businessman, in his office just hours earlier, said the governor of Nord-Kivu province, Julien Paluku. He said the reason for the killing was unclear.
Kambale specialised in imports and exports of textiles and food products, as well as in gold exports to neighbouring Uganda.
The six arrested foreigners included two Ugandans and one Kenyan, said Paluku. The nationalities of the others had not yet been determined.
Most stores and schools closed in the wake of the killing, and local residents gathered in front of Kambale's businesses before moving to the jail where the suspects were being held.
"The situation is still very sensitive in Butembo," said Paluku.
A number of prominent figures have been killed in Butemo and the nearby town of Beni since the beginning of the year.
Victims have included Raymond Kibwana, the head of the country's intelligence agency, who was shot twice in the head at his home last May.
In February, Belgian architect Claude Duvigneaud, who had made the country his home, was stabbed to death at his home.
Local politicians have regularly denounced the lack of security in Nord-Kivu region, which borders Uganda and Rwanda.
The situation is complicated by the continuing activity of the renegade Mai Mai milita groups as well as Rwandan Hutu and Ugandan rebels.