Security boosted in north Nigeria
The Nigerian authorities have stepped up security across the north of the country, following two days of violence in which at least 100 people have died.
Soldiers have set up road blocks and imposed dusk-to-dawn curfews in the worst affected areas of Yobe, Kano and Borno States.
Islamist militants staged attacks on police and government offices. There have been reports of youths armed with machetes and guns killing police officers and civilians at random.
Nigeria's military and police have been ordered to use all means necessary to contain the violence, the BBC's Caroline Duffield in Lagos reports.
Eyewitnesses told the BBC that police stations had been attacked and civilians pulled from their cars and shot dead. In the town worst affected by the violence, Maiduguri in Borno State, bodies of residents and militants have been piled outside the police station and in the streets.
A BBC reporter there counted 100 corpses.
Some of the militants are believed to be supporters of a preacher based in Maiduguri, Mohammed Yusuf, who says Western education is against Islamic teaching.
Nigeria's police are understood to be searching for him.
Late on Monday night there were still reports of shooting in Maiduguri.
Earlier, witnesses told the BBC that a battle had raged for hours in Potiskum, Yobe State, where a police station and neighbouring buildings were reportedly razed.
There was also attacks on police in Wudil, some 20km (12 miles) from Kano, the largest city in northern Nigeria.
Security is said to have been particularly beefed up in Plateau State, to the south of Bauchi, where hundreds were killed in clashes between Muslims and Christians last year.
Mr Yusuf's followers are known as Boko Haram, which means "Education is prohibited".
Youths began attacking police stations on Sunday after some of the group's leaders were arrested.
Correspondents say Boko Haram has aroused suspicion for its recruitment of young men, and its belief that Western education, culture and science are sinful.
Reuters news agency reports that one of the group's leaders, arrested in Kano state, said his followers were standing up for their faith.
"Even if I'm arrested, there are more to do the job," Abdulmuni Ibrahim Mohammed is quoted as saying.
Sharia law is in place across northern Nigeria, but there is no history of al-Qaeda-linked violence in the country.
The country's 150 million people are split almost equally between Muslims and Christians and the two groups generally live peacefully side by side, despite occasional outbreaks of communal violence.