Eid al Adha holiday began in Iraq on Tuesday with a bomb ripping through a crowd of worshippers as they left a mosque, killing 12 people.
Three children, a policeman and an army officer were among the dead from the blast in the northern city of Kirkuk, which also wounded 26 people, police and a doctor said.
Bodies, their clothes covered in blood, were placed in the back of a small police pickup truck to be taken away, an AFP journalist at the scene said.
Angry and grieving people railed against those who carried out the attack, shouting, "God take revenge on those who are evil!"
Eid al Adha, which commemorates the willingness of Abraham (Ibrahim in Arabic) to sacrifice his son at God's command, is the biggest Muslim holiday of the year.
In Iraq and other countries, people mark the holiday by slaughtering an animal such as a sheep and giving the meat to the poor.
As with various other religious occasions in Iraq, observance differs between Sunnis and Shiites.
Eid al Adha begins for Sunnis on Tuesday this year, while most Shiites consider the following day to be the first of the holiday.
On Monday, UN envoy Nickolay Mladenov had called for unity in Iraq on the occasion of the holiday.
"On this Eid and at this crucial time, I would like to plead for unity and understanding among all the Iraqis and their political, religious, and civil leaders," Mladenov said in a statement.
"It is only through working together that the people of Iraq can stand up to the violence that is tearing society apart."
Almost nothing is safe from attack by militants, and violence has reached a level not seen since 2008, when the country was just emerging from a brutal sectarian conflict.
Secure targets such as prisons have been struck in past months, along with cafes, markets, mosques, football fields, weddings and funerals.
The increasing number of attacks on both Sunni and Shiite gatherings has raised fears of a relapse into the intense sectarian bloodshed that killed tens of thousands of people in 2006-2007.
Analysts say the Shiite-led government's failure to address the grievances of Iraq's Sunni Arab minority -- which complains of being excluded from government jobs and senior posts and of abuses by security forces -- has driven the surge in unrest.
Violence worsened sharply after security forces stormed a Sunni anti-government protest camp in northern Iraq on April 23, sparking clashes in which dozens died.
And while the authorities have made some concessions aimed at placating anti-government protesters and Sunnis in general, such as freeing prisoners and raising the salaries of Sunni anti- al Qaeda fighters, underlying issues remain unaddressed.
The government has enacted new security measures, stepped up executions and carried out wide-ranging operations against militants for more than two months, but has so far failed to curb the violence.
The latest unrest takes the number of people killed so far this month to more than 310, and to over 5,000 since the beginning of the year, according to AFP figures based on security and medical sources.
In addition to major security problems, the government has failed to provide adequate basic services such as electricity and clean water, and corruption is widespread.
Political squabbling has paralysed the government, while parliament has passed almost no major legislation in years.