Nairobi/Abuja, July 31 (DPA) Nigerian police Friday hailed the killing of the leader of an Islamist sect as a victory over extremism, but human rights workers called for an investigation into his death.

Mohammed Yusuf, 39, whose Boko Haram sect caused mayhem and hundreds of deaths across northern Nigeria this week, was shot Thursday night as security forces cracked down on his followers.

'Mohammed Yusuf was killed by security forces in a shootout while trying to escape,' a police spokesman told Nigerian television.

Boko Haram, which was formed in 2002, wants to impose sharia, or Islamic law, across the whole of Nigeria and is also opposed to Western education.

Yusuf was captured after the Nigerian military stormed his compound and a mosque in the town of Maiduguri in Borno state.

Some reports suggest that as many of 100 militants were killed in the army assault. Blood-stained bodies of young men littered the streets after the battle.

The streets of Maiduguri were quiet Friday, although many members of Boko Haram were believed to be still at large.

Yusuf was paraded before local journalists after his capture. Several hours later, he was dead, supposedly shot while escaping.

New York-based Human Rights Watch called for an investigation into Yusuf's death, saying it believed it to be unlawful.

Extrajudicial killings are common in Nigeria. Police have on many occasions been accused of murder in the restive Niger Delta, where militants have been conducting a campaign of sabotage against oil facilities.