Turkish police detained four people in a hunt for the perpetrators of a car bombing in central Istanbul that killed seven police and four civilians, state media said.
The third deadly attack in Turkey's biggest city in six months targeted a bus transporting anti-riot police in Beyazit district, close to many of the city's top tourist sites, Istanbul governor Vasip Sahin said on Turkish television.
Thirty-six people were wounded, three of them seriously, he added.
The four suspects were taken to police headquarters in Istanbul for interrogation, state-run Anatolia agency said, without providing further information.
There was no early claim of responsibility, but President Recep Tayyip Erdogan suggested the outlawed Kurdistan Workers' Party (PKK) was behind the attack.
For the PKK to target major cities such as Istanbul "is nothing new," he said after visiting the injured at an Istanbul hospital.
"We will fight against terrorists relentlessly to the end."
In a statement from his office later, Erdogan also vowed the culprits would "pay the price for the blood they shed."
Kurdish militants have repeatedly targeted Turkey's security forces, but Islamic State (IS) jihadists have also staged attacks around the country, including in Istanbul, in the past year.
Reports said the explosion took place close to Vezneciler metro station, within walking distance of some of the city's main tourist sites including the Grand Bazaar and Suleymaniye Mosque.
The blast reduced the police vehicle to mangled wreckage and windows in nearby shops were shattered. Reports said that shots were heard afterwards.
The attack occurred outside the upscale Celal Aga Konagi Hotel, a converted Ottoman mansion that is favoured by foreign tourists.
The 16th-century Sehzade Mosque -- considered one of the greatest masterpieces of Ottoman architect Mimar Sinan -- was also damaged by the force of the explosion.
Television footage showed its windows blown out and debris littering the floor.
Loudspeakers on mosques warned people to vacate the area, after which a controlled explosion was carried out on a suspect vehicle.
Erdogan, who flew back to Ankara later in the day, chaired a security summit at the presidential palace, with several ministers as well as the top army general and spy chief.
French President Francois Hollande condemned the attack as an "intolerable act of violence" that should strengthen common resolve to fight terrorism.
German Chancellor Angela Merkel said Berlin was "on the side of Turkey in the fight against terrorism" and Nato chief Jens Stoltenberg also expressed solidarity.
US Ambassador to Ankara John Bass said in a Twitter message: "Such senseless violence could never be rationalised by any cause."
The United States will "continue to stand shoulder-to-shoulder with Turkey in the fight against terrorism," Bass said. EU foreign policy chief Federica Mogherini also reaffirmed support.
The bombing, which occurred on the second day of the Muslim holy month of Ramadan, is the latest in a string of attacks that have rattled citizens and damaged tourism.
Two blasts in Ankara claimed by the Kurdistan Freedom Falcons (TAK) -- a radical splinter group of the better-known PKK -- earlier this year claimed dozens of lives.