An apparent triple suicide bomb attack on a key Shia mosque in Baghdad has left 90 people dead and 160 injured, Iraqi police have said.
The carnage comes a day after warnings of attacks on mosques
The blasts happened as worshippers were leaving the Buratha mosque in the north of the city after Friday prayers.
At least two of the suicide bombers were dressed as women, police said.
This came a day after a car bomb killed at least 10 people near a Shia shrine in Najaf. Tensions have risen since a key Shia shrine was bombed in February.
That attack, on the shrine in Samarra, north of Baghdad, set off a wave of violence between Iraq's Shia and Sunni communities.
Friday's attack on the Buratha mosque is likely to increase sectarian tensions further, correspondents say.
The mosque is one of the most important for Shia Muslims in the Iraqi capital and can hold hundreds of people, the BBC's Andrew North in Baghdad says.
Its imam, Sheikh Jalaluddin al-Saghir, is a member of parliament and an important figure in Iraq's largest Shia political party, the United Iraqi Alliance.
He escaped unhurt, and later told the Arab television station al-Arabiya about the attack, the Associated Press news agency reported.
He said one assailant had entered the mosque through the women's security checkpoint and had blown themselves up first, while a second had rushed into the mosque's courtyard and a third to his office before detonating themselves.
Shias were being targeted "as part of this dirty sectarian war waged against them as the world watches silently," he was reported as saying.
Our correspondent says the attack comes a day after Iraq's interior ministry warned of possible attacks on mosques in Baghdad on Friday.
One of the most important and oldest Shia mosques in Iraq
An estimated 6,000 worshippers pray there
According to tradition, Prophet Muhammad's son-in-law visited in the 7th Century
Water from well believed to have been dug by Imam Ali held to have curative powers
Pilgrims come from as far afield as Afghanistan to drink from well
The imam, Sheikh Jalaluddin al-Saghir, is an MP
It said it had received intelligence that insurgents were preparing to detonate seven bombs around the capital.
The warning was issued as Iraqis prepared to observe a four-day weekend to celebrate the fall of Saddam Hussein on 9 April, and the birthday of the Prophet Muhammad on 10 April, the AFP news agency said.
In his Friday sermon, radical Shia cleric Moqtada Sadr blamed US troops for Thursday's bomb attack at Najaf, the agency added.
"This is not the first time that the occupation forces and their death squads have resorted to killings," the cleric was quoted as saying.
There was initial confusion after the Buratha mosque blasts, with first reports suggesting the explosions were caused by mortar fire, but police then said they believed three suicide bombers were responsible.
Rescuers carried bodies from the mosque compound on makeshift wooden handcarts and loaded them onto pickup trucks.
Patches of blood and dozens of shoes were left scattered outside the building.
Baghdad's city council urged Iraqis to donate blood for the injured.
Layla al-Khajifa, a Shia Muslim who works for the United Iraqi Alliance, told the BBC the Buratha mosque was an extremely popular place to pray on a Friday - so much so that in recent times she had not been able to get in.
"It's a very famous mosque - there is a cemetery in there," she said. "Iraqis who don't have money to go to Najaf, they bury their dead there."
She added that the mosque had played a important role in the election campaign.
The US ambassador to Iraq, Zalmay Khalilzad, said US officials had held talks with some groups linked to the Sunni-led Iraqi insurgency.
The attackers struck as worshippers were leaving the mosque
Mr Khalilzad told the BBC that he believed the talks had had an impact, as the number of attacks on US troops by Iraqi militants had fallen, but he ruled out negotiating with those he called Saddamists or terrorists.
He also urged Iraq's politicians to break the deadlock over who should lead the new government of national unity.
The current prime minister, Ibrahim Jaafari, has rejected growing pressure to resign, saying Iraqis must be allowed to choose their leader democratically.
The Buratha mosque is said to be among the most revered in Iraq.
Tradition says that Imam Ali, son-in-law of the Prophet Muhammad, visited the mosque in the 7th Century and dug a well.
The well is still in operation and attracts pilgrims who believe its waters have curative powers.