Tensions are high and security has been tightened in Kano, the largest city in northern Nigeria, where police on Monday discovered 10 car bombs and hundreds of other unexploded devices as the death toll from Friday's coordinated attacks continues to rise.

Authorities now say at least 184 people, the vast majority of them civilians, died in the bombings and shootings that have been blamed on the militant Boko Haram sect.

The Obama administration has joined in the international condemnation of the attacks that targeted mostly police stations and government buildings in Kano. A State Department spokeswoman said the United States is extremely concerned about what she termed “the horrific spate of bombings.” She said counter-terrorism experts are being dispatched to Nigeria.

On Monday, Nigerian political and religious leaders gathered to pray for peace as businesses re-opened and traffic returned to the city streets. But the mosque where the Emir of Kano, Ado Bayero, prayed was half empty. Many city residents were afraid to attend.

Kano resident Aminu Garba said people fled for cover when the tire on a passing vehicle blew out, fearing a bomb had exploded.

“We are not safe at all,” he said.

Boko Haram has said it is fighting to implement strict Sharia law across Nigeria, a country of 150 million people that is divided between the mostly Muslim north and the largely Christian south.

The ongoing violence has raised concerns that Nigeria, Africa's most populous nation and biggest oil producer, is sliding toward civil war.