Turkey said the Saturday night attack seemed to be a coordinated one on police outside a soccer stadium in Istanbul after a match between two top teams, the media reported.
India joined the international community to condemn the barbarity.
A car bomb exploded outside the Vodafone Arena, home to Istanbul’s Besiktas soccer team. It was followed by a suicide bomb attack in a nearby park less than a minute later, reports Xinhua news agency.
The ghastly attacks shook a nation still trying to recover from a series of bombings this year in cities including Istanbul and capital Ankara.
No group claimed responsibility for the attack. At least 13 people have been detained, Deputy Prime Minister Numan Kurtulmus said.
He said early indications pointed to the outlawed Kurdistan Workers Party (PKK), which has carried out a three-decade insurgency in the country.
“The arrows point at PKK,” Kurtulmus told CNN Turk.
At least 14 of the injured were in critical condition.
He said Turkey’s allies should show solidarity with the country in its fight against terrorism, a reference to the long-standing row with fellow Nato member Washington over Syria.
The US backs the Syrian Kurdish YPG in the fight against the Islamic State.
A day of national mourning was declared on Sunday and flags were at half mast, Xinhua reported.
President Tayyip Erdogan, who cancelled a planned trip to Kazakhstan, described the blasts as a terror attack.
“Nobody should doubt that with God’s will, we as a country and a nation will overcome terror, terrorist organisations … and the forces behind them,” he said.
In addition to the Kurdish insurgency, Turkey is also battling the IS as a member of the US-led coalition.
Less than a week ago, the Islamic State urged its supporters to target Turkey’s “security, military, economic and media establishment”, said the British Telegraph newspaper.
Jens Stoltenberg, the Nato Secretary-General, condemned the “horrific acts of terror”. The US said it stood with its Nato ally.
India condemned the attack “in the strongest words” and said it “stands in solidarity with the people of Turkey”.
The External Affairs Ministry said in a statement: “Terrorism in any form and manifestation is completely unacceptable.”
The bombings come five months after Turkey was shaken by a failed military coup in which more than 240 people were killed.
Istanbul has seen several attacks this year including in June when around 45 people were killed as three suspected Islamic State militants carried out a gun and bomb attack on its main Ataturk airport.