- Explosion hit near the Grand Bazaar - an area frequented by tourists
- ISIL was blamed for two bombings in Istanbul this year
- Kurdish separatists have staged similar attacks
- Tuesday's attack is the fourth major bombing in Istanbul this year
Four suspects have been detained for questioning in connection to a bomb attack that targeted a police vehicle near the main tourist district in central Istanbul, killing 11 and injuring 36 people.
The four were arrested hours after Tuesday's attack and were being held at Istanbul's main police headquarters, Turkish state media reported.
Speaking at the scene of the blast in the Beyazit district, Istanbul Governor Vasip Sahin said a bomb placed inside a car detonated as the police bus passed by, killing seven members of the police force as well as four civilians.
Reports said the explosion took place near the Vezneciler metro station, which is within walking distance of some of the city's main tourist sites, including the Suleymaniye Mosque.
The White House condemned the attack as a "horrific act" and confirmed that the US "stands together with Turkey" as the two countries confront challenges in the region.
Al Jazeera's Emre Rende, reporting from Istanbul, said the "bus was targeted by a remotely detonated car bomb before a second blast, believed to have been caused by a gas canister.
"The attack happened close to the Grand Bazaar so it might have been done to keep tourists away," he added.
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.
|Eleven people were killed and 36 wounded, three of them seriously [Reuters]
President Recep Tayyip Erdogan said on Tuesday that the country's fight against armed groups would continue "to the end". Speaking to reporters after visiting some of the injured in a hospital near the site of the blast, Erdogan said the attack was "unforgivable".
The bombing came during the first days of Ramadan, the Islamic holy month, a time marked by dawn-to-dusk fasting and intense prayer.
Istanbul had already been hit by two bombings this year, including in tourist areas.
Both attacks were blamed on the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, and a pair of attacks in Ankara that were claimed by Kurdish separatists and killed dozens.
The two attacks in Ankara were claimed by the Kurdistan Freedom Falcons (TAK) - a splinter group of the better-known outlawed Kurdistan Workers' Party (PKK).
The attacks have had a dire effect on the tourism industry 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 attack took place less than 1km from the Grand Bazaar, a popular tourist site in the Vezneciler area [EPA]
Source: Al Jazeera and agencies