The gunmen ambushed the convoy of Senator Ifeanyi Ubah of the fringe opposition Young Progressives Party at Enugwu-Ukwu community in Anambra State on Sunday, opening fire on the vehicles.
"Two police escorts, two civilian aides and a passerby were killed in the attack," Anambra State police spokesman Tochukwu Ikenga told AFP.
He said two other police escorts were wounded in the attack and they had been taken to the hospital for treatment.
Ikenga said the senator was "unhurt" during the attack, adding that an investigation was under way.
"We are on the trail of the gunmen with a view to determining their motive and bringing them to justice."
No group has claimed responsibility for the violence, just five months before February's presidential, senate and congressional elections.
Southeast Nigeria has seen a surge in violence blamed on the outlawed Indigenous People of Biafra (IPOB) group or its armed wing ESN.
IPOB, which seeks a separate state for ethnic Igbo people, has repeatedly denied responsibility for violence in the region.
More than 100 police officers and other security personnel have been killed since the beginning of last year in targeted attacks in the region, according to local media tallies.
Prisons have also been raided with scores of inmates freed and weapons stolen. Local offices of the national electoral authorities have also been targeted for attacks.
IPOB's leader Nnamdi Kanu is in government custody and faces trial for treason after he was captured overseas and brought back to Nigeria.
Separatism is a sensitive issue in Nigeria where the declaration of an independent Republic of Biafra in 1967 by Igbo army officers sparked a three-year civil war that left more than one million people dead.