As the country wondered about the health of Mr. Buhari, who has spent more than 100 days in London receiving medical treatment for a mystery illness, people poured into the streets in the southeast to support a movement to create a new state or break off into a separate nation, as the area tried to do in the late 1960s.

Last month, the military deployed tanks and helicopters in the region. Separatists say that their supporters have been killed and tortured by the military.

Troops encircled the home of the leader of the Indigenous People of Biafra, Nnamdi Kanu, and his supporters tried to fight them off with sticks. Reuters reported that bullet holes were found in the house, and that two of its journalists saw six bullet-riddled bodies in the morgue. Separatists said the six people were killed during what they said was a raid by troops on Mr. Kanu’s home. The whereabouts of Mr. Kanu is unknown.

Maj. Gen. D. D. Ahmadu, the Nigerian Army’s chief of training and operations, said the military had targeted bandits and other criminals operating in the region, adding that separatists were not being singled out. Yet another military exercise was being planned for the region, raising fears of more violence to come.

“They humiliate and dehumanize the civilian population,” said Wole Soyinka, a playwright, activist and Nobel laureate. “This type of unacceptable habit, this sense of military superiority, is beginning to creep back into this nation.”

President Buhari has spoken out harshly against the agitators. In August, he gave a stern warning to those who had “crossed the national red lines by daring to question our collective existence as a nation.”