A car bomb ripped through a police bus near Istanbul's historic centre today, killing seven officers and four civilians in the latest attack in Turkey's biggest city.
The blast targeted a shuttle service carrying anti-riot police as it passed through 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.
There was no early claim of responsibility, but President Recep Tayyip Erdogan hinted 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 in kind the price for the bloodshed."
Kurdish militants have repeatedly targeted Turkey's security forces, but Islamic State (IS) jihadists have also staged several attacks in Turkish cities 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 famed Suleymaniye Mosque.
The metro station was closed as a security precaution.
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 opposite an upscale hotel favoured by foreign tourists, the Celal Aga Konagi Hotel, a converted Ottoman mansion.
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.
French President Francois Hollande condemned the attack as "intolerable act of violence" that should strengthen common resolve to fight terrorism.
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.
