A bomb ripped through a Turkish police vehicle near Istanbul's historic centre Tuesday, killing seven officers and four civilians and adding to security concerns after a string of attacks in Turkey's biggest city.

The bomb targeted a service shuttle bus carrying officers from Istanbul's anti-riot police as it was passing through the central Beyazit district close to many of the city's top tourist sites, Istanbul governor Vasip Sahin said in a live statement on Turkish television.
Thirty six people were wounded, three of them seriously, he added.
Reports said the explosion took place close to the Vezneciler metro station, which is 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.
Pictures showed the bomb had turned the police vehicle into mangled wreckage and that nearby shops had their front windows smashed out by the force of the blast.
Cars parked in the vicinity were also damaged.
Television pictures showed bomb disposal experts examining the scene in case of a second unexploded bomb and reports said at least one controlled explosion was carried out.
Scheduled examinations at Istanbul University -- which lies close to the scene of the blast -- have been cancelled.
Reports said that shots were heard and pictures showed police in bullet proof vests brandishing their weapons.
The blast took place opposite an upscale hotel favoured by foreign tourists, the Celal Aga Konagi Hotel, a converted Ottoman mansion.

Year of attacks

There was no immediate indication of who had carried out the attack.
But since the start of the year, Turkey has been hit by a sequence of attacks that have rattled citizens and also caused tourism to plummet.
Two separate blasts in Ankara claimed by the Kurdistan Freedom Falcons (TAK) -- a radical splinter group of the better-known outlawed Kurdistan Workers' Party (PKK) -- earlier this year claimed dozens of lives.
Last month, at least eight people including soldiers were wounded by a remotely detonated car bomb aimed at a military vehicle in Istanbul that was claimed by the PKK.
Meanwhile, a dozen German tourists were killed on January 12 in a bombing in the heart of Istanbul's tourist district blamed on Islamic State jihadists.
At least three Israelis and an Iranian were killed in a March 19 bombing on Istanbul's main Istiklal shopping street which was also blamed on IS jihadists.
The attacks have also come as Turkey is battling PKK militants, who have themselves killed hundreds of members of the security forces in the southeast.
The government has warned there will be no let up in the fight until the PKK is defeated and the militants have in turn threatened more attacks.
But the attacks in Turkey's heartland have had a dire effect on the tourism industry and further violence in Istanbul comes at the worst possible time heading into the key summer season.
Some 1.75 million foreigners came to Turkey in April, down more than 28 percent on April 2015, the tourism ministry said in its latest release.
The fall was the steepest monthly decrease for 17 years and raised fresh concerns about the health of the industry heading into the crucial summer season.
The US embassy in Turkey in April warned of "credible threats" to tourist areas in Istanbul and the resort city of Antalya, in particular to public squares and docks.
Turkey, a member of NATO and the US-led anti-jihadist coalition, appears to have stepped up its operations against IS in northern Syria, where the extremists control areas near the border, which some analysts say has made it more vulnerable to attack.