Amnesty International said it had "strong evidence" that police and soldiers used automatic weapons against members of the Islamic Movement of Nigeria (IMN) at marches in and around Abuja.
The movement of a jailed Nigerian Shia cleric whose followers have repeatedly been targeted by the authorities said on Wednesday security forces had killed 42 of its members during two days of violent crackdowns on protests in the capital Abuja.
Security forces opened fire with live ammunition on members of the Islamic Movement of Nigeria (IMN) who had marched in their hundreds to demand the release of their leader Ibrahim Zakzaky, jailed since 2015 when the army killed hundreds of his followers at his compound and a nearby mosque and burial ground.
The IMN raised its death toll on Wednesday from the two previous days' violence to 42 from an earlier figure of 25. The toll included seven people who died of injuries received on Tuesday and 35 killed the previous day, said Ibrahim Musa, an IMN spokesman.
On Wednesday evening, 20 bodies lay shrouded in white, awaiting burial in the town of Mararaba, some eight km [five miles] from Abuja, a Reuters reporter said.
"I, as a Shia today, I'm carrying my death certificate," said Muhammed Ibrahim Gamawa, an IMN youth leader, at the burial site.
"Under Buhari's government any Shia in Nigeria is under threat and can be gunned down any day, any time, any minute," he said, referring to Nigerian President Mohammadu Buhari.
"We are not safe, we are an endangered species in Nigeria."
A few hours earlier, in a bustling commercial area of Abuja, dozens of IMN members staged a short march.
Security forces were absent.
"We are ready to die for our Zakzaky!" and "Free Sheikh Zakzaky!" the marchers chanted.
"Death to Buhari! Death to America! Death to UK!" they also cried. Buhari's administration has overseen the deadly crackdown on IMN over the past three years.
Protests met by force
On Monday, the army opened fire on the marchers on the outskirts of Abuja. On Tuesday, the police shot at the protesters in the city centre, issuing a statement later that day saying 400 IMN members were detained.
The statement did not mention any deaths.
Police and army spokesmen did not respond to phone calls seeking comment on the updated death toll. The army has said that three IMN members were killed on Monday.
Zakzaky was charged in April this year with murder over the 2015 violence, after being held for more than two years. Authorities ignored a court ruling during the period before he was charged that he be released, sparking protests from his followers.
IMN protests have frequently been met with force. In April, police fired bullets and tear gas during days of protests by IMN, wounding at least four protesters.
After the December 2015 crackdown, Buhari accused the Shias of creating "a state within a state," though he also said civilian deaths could not be justified. Since then, however, the government has remained largely silent on accusations it has used excessive force against the group.
Around half of Nigeria's 190 million people are Muslims. Although virtually all of them are Sunnis, Zakzaky has attracted an estimated three million followers as a preacher of Shia Islam since being drawn to that sect by the 1979 revolution in Iran.
The repression of IMN and detention of its leader have drawn criticism from international human rights watchdogs and raised concern that the group could become radicalised.
Amnesty accuses Nigerian forces
Human rights campaigners on Wednesday accused Nigeria's security forces of killing dozens by using "horrific" force during a crackdown on protesting supporters of an imprisoned Shia cleric.
Amnesty International said it had "strong evidence" that police and soldiers used automatic weapons against members of the IMN at marches in and around Abuja.
The group said upwards of 45 people were killed – six on Saturday and at least 39 on Monday, when some 122 were also injured.