An IS suicide bomber as young as 12 attacked an outdoor Kurdish wedding party in south-eastern Turkey, killing at least 51 people and wounding dozens of others, the Turkish president said.
The bombing late Saturday in Gaziantep, near Turkey's border with Syria, was the deadliest attack in Turkey this year.
President Recep Tayyip Erdogan, speaking live on national television in front of Istanbul's city hall, said the attacker was aged between 12 and 14. He said 69 people were wounded, 17 of them in a critical condition.
"It was clear that Daesh had such an organisation in Gaziantep or was attempting to make room for itself in recent times," Mr Erdogan said, using an alternative acronym for IS.
"Many intensive operations were conducted, are being conducted. Of course our security forces will be conducting these operations with even greater intensity."
A bus driver who shuttled some of the guests from Siirt to Gaziantep said that he could not believe the party was targeted.
"This was a wedding party. Just a regular wedding party," Hamdullah Ceyhan told the state-run Anadolu Agency. "This attack was deplorable. How did they do such a thing?"
Turkey has been rocked by a wave of attacks in the past year that have either been claimed by Kurdish militants linked to the outlawed Kurdistan Workers' Party - known by its acronym PKK - or were blamed on IS.
In June, suspected IS militants attacked Istanbul's main airport with guns and bombs, killing 44 people. A dual suicide bombing blamed on IS at a peace rally in Turkey's capital, Ankara, in October killed 103.
The attack comes as the country is still reeling from last month's failed coup attempt, which the government has blamed on US-based Muslim cleric Fethullah Gulen and his followers. Gulen denies any involvement.
Earlier, Mr Erdogan said there was "absolutely no difference" between IS, Kurdish rebels and Gulen's movement, calling them terrorist groups.
"These bloodthirsty organisations and the powers behind them have neither the will nor power to silence the calls to prayer, lower the flag, divide our motherland and break up our nation," he added.
Earlier this week, a string of bombings blamed on the PKK that targeted police and soldiers killed at least a dozen people. A fragile, two-and-a-half-year peace process between the PKK and the government collapsed last year, leading to a resumption of the three-decade-long conflict.
In Gaziantep, deputy prime minister Mehmet Simsek and the country's health minister visited the wounded and inspected the site of the attack.
"This is a massacre of unprecedented cruelty and barbarism," he told reporters. "We are united against all terror organisations. They will not yield."
Prime minister Binali Yildirim condemned the bombing, which he said turned "a wedding party into a place of mourning" and he vowed to prevail over the "devilish" attacks.
Opposition parties have also denounced the attack. The main opposition Republican People's Party will be holding an emergency meeting and a delegation was being sent to Gaziantep by the Nationalist Movement Party.
Supporters of the pro-Kurdish People's Democracy Party will be holding a protest against the attack in Istanbul.
Foreign governments, including the US, Sweden, Greece, France, Bahrain, Qatar and Jordan, have condemned the attack.
Police sealed off the site of the explosion and forensic teams moved in.
Hundreds of residents had gathered near the site chanting "Allah is great" as well as slogans denouncing attacks.
Turkish authorities issued a media blackout on coverage of the attack until the investigation is completed.
The bride and groom were not in a life-threatening condition and were undergoing treatment, Anadolu reported. The groom's sister and uncle were among the dead, the agency said.
German chancellor Angela Merkel has sent a telegram of condolence to Turkey, saying Germany stands on Turkey's side in the "fight against terrorism."
Mrs Merkel said it was with her "deepest sorrow" that she learned of the attack.
"Once again innocent men, women and children are the victims of cowardly and perfidious violence; I condemn this attack in the strongest terms," she said.
Mrs Merkel said Germany's thoughts are with the victims and their families, and she wished speedy recoveries to those wounded.
"I assure you that the German government will continue to stand closely at Turkey's side in the fight against terrorism," she added.