Nigeria's government has taken the controversial decision to ban a pro-Iranian Shia group, accusing it of unleashing violence and being an "enemy of the state".
The Islamic Movement in Nigeria (IMN) is challenging the ban, arguing that it is a peaceful movement which has, in fact, borne the brunt of state-orchestrated violence.
These developments have raised fears of oil-rich Nigeria becoming the latest battleground in the conflict between the world's two main Muslim sects, Shia and Sunni.
On Monday, a court in Kaduna in northern Nigeria, ruled that the group's founder and leader Sheikh Ibraheem Zakzaky - who has been in police detention since 2015 - should be allowed to travel to India to receive medical treatment. He has been suffering minor strokes and is reported to be losing his sight.
What is the IMN?
Formed about four decades ago, it advocates the creation of an Iranian-style Islamic state in Nigeria.
It was heavily influenced by the Iranian revolution, which saw Ayatollah Khomeini take power in 1979 after the overthrow of the US-allied Shah in a popular uprising.
Khomeini remains the group's main inspiration: IMN supporters first pledge allegiance to him at their gatherings, and then to their local leader, Sheikh Zakzaky.
The IMN views itself as a government, and Sheikh Zakzaky as the only legitimate source of authority in Nigeria.
It does not recognise the authority of the Nigerian government, and views its leaders - both Muslims and Christians - as corrupt and ungodly.
It has well-organised branches and administrative structures in most of Nigeria's 36 states to give it the semblance of a government.
The IMN also operates its own schools and hospitals in some states in northern Nigeria, where most Muslims live.
"The Islamic Movement has a registered Foundation called the Fudiyya Foundation under which there are over 360 primary and secondary schools. This largely reduces the burden on the government of its inability to provide basic education to the populace," it says on its website.
Many of its members are known to be well-educated professionals, and some of them hold posts in the the army, police and intelligence agency.
Jacob Zenn, a US-based analyst with The Jamestown Foundation think tank, argues that the presence of IMN members in the civil service shows that it supports an "Islamic evolution", rather than revolution, in Nigeria.
"Despite maintaining a lightly armed 'Hizbollah-like' guard corps, a newspaper, and pro-Khomeini imagery on the IMN website and at demonstrations, the IMN has more recently settled for highly public, and even ostentatious, Shia rituals as its hallmark, as opposed to direct political agitation," he wrote in an article published on 26 July.
How popular is the IMN?
The group is the largest Shia organisation in Nigeria.
It can draw huge crowds - sometimes in the tens of thousands - at its gatherings.
This is largely because of the efforts of Sheikh Zakzaky, who spurred the growth of Shiaism in a country where there were a negligible number of Shia before the Iranian revolution.
Estimates of their numbers vary wildly, ranging from less than 5% to 17% of Nigeria's Muslim population of about 100 million.
Most Nigerian Muslims are Sunni, like those in Saudi Arabia or Egypt.
What is the difference between Shia and Sunni?
Divisions between the two sects are ancient, going back to the leadership dispute which arose after the death of Prophet Muhammad in the early 7th Century.
Some Muslims called for his son-in-law, Ali, to succeed him, while the majority backed his father-in-law, Abu Bakr.
The dispute continued to rage long after the deaths of both and supporters of Ali eventually became known as Shia, creating what remains the oldest and biggest schism in Islam.
For Sunni Muslims, Abu Bakr and his supporters are pious role models, while many Shia - though not all - see them as traitors.
The differences between the two sects continue to dominate religious life, and spill over into other aspects as well. In Nigeria for example, it is extremely rare to hear of a Sunni and Shia marrying each other.
So why has the government banned the IMN?
Nigerian President Muhammadu Buhari's office said the IMN had been banned because it had been "taken over by extremists who didn't believe in peaceful protests and instead employed violence" to achieve its objectives.
This follows months of protests by the group to demand the release of Sheikh Zakzaky, who has been in detention since 2015, and justice for the hundreds of its members killed by the security forces.
Many of these protests have seen violent clashes between protesters and the security forces, including outside the federal parliament building.
A senior police officer was among those killed in the capital, Abuja.
"Any person engaged or associating, in any manner that could advance the activities of the proscribed Islamic Movement in Nigeria IMN, shall be treated as a terrorist, enemy of the state, and a subversive element and shall be brought to justice," police chief Mohammed Adamu told senior officers after the order was granted.
What has the IMN said about the ban?
The IMN denies being behind the violence, and accuses the security forces of killing several peaceful protesters.
It sees Mr Buhari - a Sunni Muslim - as a pawn of Saudi Arabia, who it says wants to eliminate them.
It is challenging its banning in court, and says it will halt street protests to give mediators a chance to resolve the "crisis" over Sheikh Zakzaky's detention.
He has remained in custody despite the fact that the federal high court ruled in December 2016 that his detention was unlawful. He is being held along with his wife, Zeenah.
The government denies it is defying a court ruling, saying the couple are in "protective custody".
Sheikh Zakzaky, 66, is said to have had a stroke earlier this year, while his wife is said to be suffering from severe osteoarthritis and hypertension.
There is no doubt that the couple have been going through a tough time. The IMN says three of their sons and Sheikh Zakzaky's eldest sister were killed in a security force crackdown in a 2015 on Sheikh Zakzaky's residence and headquarters in Zaria city in Nigeria's northern Kaduna state.
Why were the couple arrested?
The army alleges it was forced to retaliate after IMN members - who were attending a religious ceremony in Zaria - tried to kill its chief of staff, who was driving through.
"The troops responsible for the safety and security of the Chief of Army Staff, on hearing explosions and firing, were left with no choice than to defend him and the convoy at all cost," it said in a statement.
The IMN's headquarters was demolished, about 350 people were killed in the operation, and hundreds of people were arrested - including Sheikh Zakzaky who was accused of murder, manslaughter, unlawful assembly and disruption of public peace. He denied the charges.
The security force operation sparked outrage, with critics drawing parallels between the crackdown on the IMN and Boko Haram, a Sunni insurgent group in northern Nigeria.
Could the IMN start a rebellion?
Troops raided Boko Haram's headquarters in the north-eastern city of Maiduguri in 2009, resulting in clashes that left more than 700 people dead. Its founding leader, Mohammed Yusuf, was arrested and killed in police custody.
Boko Haram then launched a full-scale insurgency, which has claimed nearly 30,000 lives and has left more than two million homeless.
Some Nigerians fear that in the wake of its banning, the IMN could turn into a Boko Haram-style insurgent group, and cause greater havoc because it has members in most Nigerian states, unlike Boko Haram which is confined to the north-east, as well as many followers in the security forces.
Others rule out this possibility, pointing out that IMN members tend to be well-educated and would not risk a war in which there would be no winners.
Mr Zenn agrees with the latter view, saying there "does not appear to be any organized insurgency in the works".
He does not think Iran would try to stir up trouble in Nigeria.
"Even if Iran wanted to back a proxy militia in Nigeria, the geographic and cultural distance would hinder it.
"In addition, Iran is overstretched in Middle Eastern conflicts and has less commitment to Zakzaky than it may appear on the surface, especially given the other Nigerian Shia factions," he said.
How have others reacted to the ban?
Human Rights Watch (HRW) condemned the ban, saying it "threatens the basic human rights of all Nigerians".
"The ban on the Shia movement may portend an even worse security force crackdown on the group, which could have dire human rights implications throughout Nigeria," said HRW Nigeria researcher Anietie Ewang.
The Catholic cardinal of Abuja also condemned the ban.
"Nobody is safe: today it's the Shia, tomorrow it could be us Catholics too," Cardinal John Onaiyekan told Vatican Radio.
The government dismisses such suggestions, saying everyone - including IMN supporters - are free to practise their religion.
Has Iran said anything?
Protests have repeatedly been held in Tehran and elsewhere to demand Sheikh Zakzaky's release, while President Hassan Rouhani phoned his Nigerian counterpart after the 2015 killings to protest.
But Iran has not publicly condemned the banning of the IMN, possibly because it does not want to jeopardise mediation efforts to secure the release of Sheikh Zakzaky and his wife.