ABUJA, Nigeria — More than 100 people have been killed in a series of attacks in northern Nigeria’s largest city in what appears to be the deadliest strike yet by a radical Islamist group.

The attackers in Kano on Friday evening struck eight government security buildings, the national police said, including the regional police headquarters, two local police stations, the local headquarters of the State Security Service, the home of a police official and the state police command headquarters.

The radical sect Boko Haram, which has carried out an escalating campaign of violence in its battle to impose its version of Islamic law across Nigeria, claimed responsibility. A letter distributed to reporters on Saturday said the attacks in Kano, Nigeria’s second-largest city, were retribution for the arrests and killings of members of the sect.

Residents in Kano described bloody scenes of chaos and confusion as bombs exploded and gunmen started shooting in the street.

At least 143 people had been killed.

President Goodluck Jonathan, who has been criticized for failing to act against Boko Haram, which has killed hundreds of Nigerians in the past year, said in a statement that he was “greatly saddened” by the attacks and that he promised that “all those involved in that dastardly act would be made to face the full wrath of the law.”

In Kano, where state authorities declared a 24-hour curfew, shellshocked residents stayed in their homes.

Kano, a city of more than nine million people, is a major political and religious center in the predominantly Muslim north. About half of Nigeria’s 160 million people are Muslim.

Boko Haram, whose name means “Western education is forbidden” in the local Hausa language, has focused its attacks mainly on government and police sites in the north, and has also threatened to kill any Christians living there. The group carried out a series of attacks on churches last Christmas. Last summer, the sect appeared to broaden its focus when it attacked the national police headquarters in the capital, Abuja, and the United Nations building there, killing at least 23 people.

“Unless somebody goes in to negotiate,” he said, “we are in for a long siege.”

Whatever their doubts, most democrats in the Arab world reckon that Islamists who say they will abide peacefully by the rules of the game must be allowed—indeed encouraged—to participate in mainstream politics: far better than forcing them into a violent, conspiratorial underground. All the same, the well of mistrust on both sides runs deep.

Many liberals still think the Islamists, however mild they sound today, are bent on taking over in the long run, would abandon democracy once they got into power and would use every sort of chicanery and violence to achieve their goal.

Two articles on the relationship between Islam and democracy in light of the 2011 “Arab Spring”