At least 11 people have died in clashes in northern Nigeria as Shia Muslims mark an important religious event.
Ashura day gatherings, which commemorate the killing of the grandson of the Prophet Muhammad, were banned in some places, including Katsina state.
Witnesses told the BBC that nine people died in the state's Funtua town when troops tried to block the march.
Police say unruly crowds also attacked other Shia gatherings, including in Kaduna city where two people died.
A house belonging to a leader of pro-Iranian Islamic Movement of Nigeria (IMN) Shia sect was also set alight in the city.
Kaduna state government outlawed the IMN last Friday, saying it was a threat to the state.
Most of Nigeria's Muslims are Sunnis, and there are underlying tensions between them and Shia Muslims.
The UK-based Islamic Human Rights Commission (IHRC) said in a statement that security forces opened fire with live rounds on followers at the procession in Funtua.
In the northern city of Kano, police say as Shia mourners were returning home after their procession they were set upon by young men - and officers had to rescue 100 of them from their attackers.
In Jos, the main city of the central state of Plateau state, eyewitnesses says an Islamic Shia centre set on fire.
Shia in Nigeria:
- Shia are minority in Nigeria but their numbers are increasing
- The IMN, formed in the 1980s, is the main Shia group
- They operate their own schools and hospitals in some northern states
- They have a history of clashes with the security forces
- The IMN is backed by Shia-dominated Iran and its members often go there to study
- Sunni jihadist group Boko Haram condemns Shias as heretics who should be killed
The IHRC says that this year's Ashura processions were preceded by a military build-up that was "reminiscent" to the run-up of clashes last December when 349 Shia Muslims were killed.
In August, a judicial inquiry said troops should be prosecuted for those killings which took place in Zaria in Kaduna state.
The military previously blamed the unrest on the Shia sect, accusing its members of trying to assassinate the army chief - allegations the IMN denied.
IMN's leader Sheikh Ibraheem Zakzaky, who was shot and injured in the December clashes, remains in custody.