Kano - President Goodluck Jonathan vowed Saturday to hunt down those behind "heinous" attacks which left at least 120 dead at the mosque of an Islamic leader who had issued a call to arms against Boko Haram.
At least 270 others were also wounded when two suicide bombers blew themselves up and gunmen opened fire during weekly prayers on Friday at the Grand Mosque in Kano, the biggest city in the mainly Muslim north of the country.
Jonathan had "directed the security agencies to launch a full-scale investigation and to leave no stone unturned until all agents of terror ... are tracked down and brought to justice," said a statement from his office on Saturday.
"The president reaffirms that terrorism in all forms ... is a despicable and unjustifiable threat to our society."
The mosque is attached to the palace of the Emir of Kano Muhammad Sanusi II, Nigeria's second most senior Muslim cleric, who last week made a call at the same mosque, urging civilians to take up arms against Islamist extremists Boko Haram. The Emir was out of the country during the attacks.
The attack was widely seen as revenge for the call.
"It was death and blood all over. People lay dead and others shrieked in horror and pain," one survivor, Muhammad Inuwa Balarabe, told AFP from his hospital bed on Saturday.
"I was inside the premises of the mosque. As soon as the prayer started, a bomb went off. They just started shooting people," said the 32-year-old tailor, who received serious burns to his thighs.
Jonathan urged Nigerians "not to despair in this moment of great trial in our nation's history but to remain united to confront the common enemy."\
"One wonders what kind of religion these people practise," said survivor Maikudi Musa, who lost one sibling in the blast and saw another badly hurt.
"You can't justify attacking and killing defenceless people at will in the name of religion."
Just hours before the Kano massacre, a suspected remote-controlled roadside bomb near another mosque nearly 600 kilometres (375 miles) away in Maiduguri, was defused.
Maiduguri, where Boko Haram was founded in 2002, was already tense after two women female suicide bombers wreaked havoc at a crowded market on Tuesday, killing more than 45 shoppers and traders.
Mass casualties from Boko Haram attacks are not however a new phenomenon in the extremists' five-year insurgency. More than 13,000 people are thought to have died in total since 2009.
'Out of control Boko Haram'
After the latest attacks, the special representative of the UN Secretary-General for West Africa, Mohamed Ibn Chambas, called on Nigerian authorities "to increase their response against terrorist threats in north-eastern Nigeria", and for additional measures to protect civilians.
"Out of control Boko Haram kills dozens in Kano Mosque," read one headline on Saturday, in the influential private newspaper ThisDay.
Security expert Ona Ekhomu told a TV debate that the latest attacks showed that Africa's most populous country was at war.
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"This is mass murder, genocidal killing of people. We are at war in Nigeria," he told a local TV programme. "We have an ongoing warfare and we are not addressing it. We are distracted by politics."
In the same programme, national police spokesman Emmanuel Ojukwu said: "We try to prevent crimes from happening...but criminals sometimes beat the security."
With northern Nigeria gripped by fear, neighbouring Cameroon, Niger and Chad are also concerned that the violence could spread across their borders.