Dozens have been killed in a gun and bomb attack during prayers at one of the biggest mosques in the northern Nigerian city of Kano, officials say.
Many more people have been hurt, with one rescue official putting casualty figures at almost 400.
The Central Mosque is where the influential Muslim leader, the Emir of Kano, usually leads prayers.
The emir recently called for people to arm themselves against Islamist militant group Boko Haram.
No group said it had carried out the attack, but the assumption is that Boko Haram was behind it.
The group has been waging an insurgency in Nigeria since 2009 and has killed more than 2,000 people this year, rights groups say.
President Goodluck Jonathan condemned the attack, calling on all Nigerians "to remain united to confront the common enemy".
He said the government would "continue to take every step to put an end to the reprehensible acts of all groups and persons involved in acts of terrorism".
Nigerian police said 35 people were killed in the attack, but some eyewitnesses said far more people lost their lives.
The rescue official, speaking to Agence France-Presse, put the casualty toll at 120 dead and 270 hurt, although this has not been independently confirmed.
Three bomb explosions were reported in and around the mosque. The attackers also turned gunfire on worshippers.
Some reports say the first bomb was hidden in a car which was driven straight into the worshippers.
One eyewitness told the BBC's Focus on Africa: "The imam was about to start prayer when he saw somebody in a car trying to force himself into the mosque. But when people stopped him, he detonated the explosions. People started running helter-skelter."
There was pandemonium as people ran for their lives.
But then several men then opened fire on the crowd killing more people. Three of the gunmen were caught, and - as the terror turned to rage - they were killed on the spot, the BBC's Will Ross in Abuja reports.
BBC Hausa editor Mansur Liman said one witness at a local hospital had described the scenes there as being the most horrible he had ever seen.
Analysis: Tomi Oladipo, BBC Africa security correspondent, Lagos
It is clear it is not only Christians who face the threat of violence in northern Nigeria. This is a major mosque, frequented by one of the country's most influential Muslim leaders, Kano's emir.
Emir Muhammad Sanusi II has criticised Boko Haram and only last week urged civilians to take up arms against the group. This has raised questions as to whether he was the target of today's attack - although he was out of the country at the time.
While violence in remote rural areas is no longer news for many Nigerians, the increasing attacks in larger cities are sending shockwaves across the country. The military's ongoing counter-terrorism efforts will need more concrete results to boost public confidence.
Our correspondent says Boko Haram will be the main suspects, as the attack bore all the hallmarks of the group.
No-one from the group has yet commented.
Boko Haram has stepped up attacks against civilian targets since the Nigerian military launched an offensive last year.
Boko Haram was also behind the kidnapping of more than 200 schoolgirls from Chibok in Borno state this year, an act that sparked international outrage.
Who are Boko Haram?
- Founded in 2002
- Initially focused on opposing Western education - Boko Haram means "Western education is forbidden" in the Hausa language
- Launched military operations in 2009 to create Islamic state
- Thousands killed, mostly in north-eastern Nigeria - also attacked police and UN headquarters in capital, Abuja
- Some three million people affected
- Declared terrorist group by US in 2013
'Call for anarchy'
The emir, Muhammad Sanusi II, is currently in Saudi Arabia.
Reliable sources in the emir's palace told the BBC that he had arrived in Saudi Arabia late on Thursday night from Paris.
Boko Haram has targeted the city, the largest in northern Nigeria, several times during its insurgency.
But most of its attacks are further east - in Borno and neighbouring states.
Earlier this month, the emir told a prayer meeting that people should "acquire what they need" to protect themselves.
A police spokesman said the emir's comments were a "call for anarchy" and should not be acted on.
The emir, who until earlier this year was governor of Nigeria's central bank, normally stays silent on political matters.