Hawzah News Agency (Abuja, Nigeria) - The protesters were demanding the release of Ibrahim Zakzaky. He has been in prison since 2015, when the Nigerian army killed hundreds of his supporters near his home in Zaria. In April, he was charged with murder.
The Nigerian military said its troops were acting in self-defence, although there were no reports of casualties among the security forces.
Defence spokesman, John Agim, said: "They met the soldiers in the call of their duty and the troops tried to defend themselves.
However, some of the dead had been shot in the chest and head, and others had multiple gunshot wounds.
Osai Ojigho of Amnesty said: "It seems the Nigerian military are deliberately using tactics designed to kill when dealing with Islamic Movement of Nigeria (IMN) gatherings. Many of these shootings clearly amount to extrajudicial executions.
"This violent crackdown on IMN protesters is unjustified and unacceptable. They were perfectly within their rights to hold a religious procession and protest and there was no evidence they posed an imminent threat to life."
British taxpayers' money is spent on training Nigerian troops in a long-term arrangement, which was bolstered by the security agreement Theresa May signed with the country's president, Muhammadu Buhari, on her trip to Africa in August.