One of the leaders of Nigerian Islamist militant group Boko Haram has been arrested after a shoot-out in the northern city of Kano, police say.Muhammed "Hamza" Aliyu was held as he tried to flee the city after an exchange of fire outside his residence, they said.Three police officers and four militants were killed, according to Kano state police chief Ibrahim Idris.Boko Haram has staged numerous attacks across northern and central Nigeria.The group, whose name means Western education is forbidden, often targets the security forces and state institutions.In August it claimed responsibility for a bomb at the UN headquarters in Nigeria's capital, Abuja, which killed at least 23 people.