Nigerian government officials said Mohammed Yusuf, 39, was shot while trying to escape. His capture by police had been announced just hours earlier.
His group is blamed for days of unrest that has left hundreds of people dead....
Staff at Human Rights Watch said there should be an immediate investigation into the case.
"The extrajudicial killing of Mr Yusuf in police custody is a shocking example of the brazen contempt by the Nigerian police for the rule of law," said Human Rights Watch's Eric Guttschuss.
Another Human Rights Watch researcher, Corinne Dufka, told AP news agency: "The Nigerian authorities must act immediately to investigate and hold to account all those responsible for this unlawful killing and any others associated with the recent violence in northern Nigeria."'
Is this deliberate provocation? The houses torched by
It almost defies belief. Do these smart, smart people really think that Northern Nigeria has a crime problem? Do they really think that the missing ingredient is proper police procedures?
Think back to Rwanda. What stopped the Hutu genocide? A highly effective Tutsi army. They didn't go round the country insisting that the Hutus follow proper police procedures; there wouldn't be any Tutsis left. Is the HRW attitude parochial and ignorant, or is it worse than that?
Given their casual equanimity with hundreds of brutally murdered Christian Nigerians, we have to suspect that it is.