United Nations forces have freed a key ally of DR Congo's President Joseph Kabila from a radio station using a tank after an armed stand-off.
At least five people were also killed in rioting at Kinshasa's main prison
Nzanga Mobutu, son of late Congolese ruler Mobutu Sese Seko, was campaigning on behalf of President Kabila in Gbadolite, in the north of the country.
Four people were reported killed in rival faction violence in the town.
It is the last day of campaigning ahead of Sunday's election. The capital, Kinshasa, is reported to be calm.
Mr Mobutu was escorted to Gbadolite airport by UN soldiers after being released and flew off in good health.
A regional spokesperson for the United Nations force told the BBC that Thursday's events were unlikely to have an impact on the presidential run-off.
Gbadolite is a remote location surrounded by jungle in the far north of the country.
United Nations sources in the area say three policemen and a soldier loyal to rival presidential candidate Jean-Pierre Bemba were killed, and one of Mr Mobutu's guards was injured.
Gunfire erupted when troops loyal to Mr Bemba surrounded the headquarters of a radio station, while Mr Mobutu was inside, a UN spokesman said.
It is unclear why Mr Mobutu was visiting Radio Liberty, which is owned by Mr Bemba.
Mr Mobutu's armed guards had occupied the radio headquarters and fired at journalists, said a spokesman for Mr Bemba in a statement to AFP news agency.
Gbadolite is the Mobutu family's home town but Mr Bemba has strong support there. The two candidates were belligerents in the civil war and both still have loyal armed forces.
The stand-off in Gbadolite comes just hours after rioting broke out at the main prison in the capital, Kinshasa.
A source inside the prison and a human rights groups have told the BBC's Arnaud Zajtman that five inmates were killed in the violence.
Armed police restored order after several hours of rioting. The police say no-one was killed and 10 people were injured, nine of them lightly.
The unrest started after relatives were stopped from bringing food to prisoners after a breakout earlier in the week.
Fourteen men who escaped on Monday had been sentenced to death for their role in the 2001 assassination of former President Laurent Kabila.
Sunday's run-off vote will conclude the country's first fully democratic polls since independence in 1960.