The terrorists attacked the city “with rocket-propelled grenades and multiple suicide bombers,” witnesses said on Monday.
At least 50 people were killed and scores more wounded in overnight blasts and shootouts and a bombing outside a mosque at dawn.
The military said there were multiple attacks at four southwestern entry points to the city.
A soldier said the militants had fired rocket-propelled grenades into four residential areas on the outskirts of the city.
Soldiers fired back, and many civilians were caught in the crossfire, according to the soldier, who spoke on condition of anonymity because he was not authorized to speak to journalists.
In another blast, two girls blew themselves up in Buraburin neighborhood, killing several people, according to civil servant Yunusa Abdullahi.
"We are under siege," Abdullahi said. "We don't know how many of these bombs or these female suicide bombers were sneaked into Maiduguri last night."
Meanwhile, medics say the death toll could go higher.
A nurse at Maiduguri Specialist Hospital said dozens of critically wounded, mainly children and women, may not survive. The nurse, who also spoke on condition of anonymity, said the hospital was overflowing with patients. About 60 people had wounds from bullets and shrapnel from explosive devices, she said. Other wounded people had to be sent to other hospitals in the city.
Maiduguri, the city under attack, is the birthplace of the Takfiri Boko Haram militant group.
A separate twin bombing attack at a market near a busy bus station killed at least 30 people in Madagali, a town in Adamawa State 150 kilometers (95 miles) southeast of Maiduguri, witnesses said. The commanding officer in northeast Adamawa, Brigadier General Victor Ezugwu, confirmed the attack, but said the number of casualties has yet to be confirmed.
The multiple attacks appears to be a challenge to President Muhammadu Buhari's declaration last week that Boko Haram has been "technically" defeated, capable of no more than bombings on soft targets.
About 20,000 people have been killed in Nigeria and hundreds others elsewhere as the Boko Haram Takfiri militants, affiliated to Daesh terrorists, have extended their attacks across its borders into Cameroon, Niger and Chad.