A suspected bomb blast hit a police vehicle on Tuesday near a bus stop in central Istanbul, leading to a number of deaths.

A remote-controlled bomb exploded as a service shuttle carrying police officers was passing in the Beyazit district, state-run TRT television reported.

Police detained four people in connection with the bombing, Turkish news agencies reported. The suspects were being questioned at Istanbul's police headquarters.

Turkey's Dogan news agency reported that the four people rented the car which was used in the attack.

Eleven people were killed in the explosion, including seven police officers, Istanbul Governor Vasip Sahin confirmed shortly after the attack. In addition, he said that at least 36 people had been injured.

Ambulances and fire engines were sent to the scene, images of which showed a city street whose storefronts had been shattered by the force of the blast. Gunshots had also been heard in the area following the blast, according to state-run news organization Anadolu Agency.

The explosion took place close to the Vezeciler metro station, which is within walking distance of some of the main sights of the historical center, including the Suleymaniye Mosque. It is also close to an Istanbul University building and Istanbul's municipality building.

No group has claimed responsibility for the attack.

Shortly after the attack, President Recep Tayyip Erdogan visited some of those who were wounded. "These [attacks] are being carried out against people whose duty it is to ensure the security of our people. These cannot be pardoned or forgiven. We shall continue our fight against terrorists tirelessly until the end," Erdogan later told reporters.

Following the attack, Germany's Chancellor Angela Merkel said she was "horrified" about the attack and voiced support for Turkey, a country with which Germany has close - albeit increasingly fraught - relations.

"When it comes to the fight against terrorism, Germany is on Turkey's side," Merkel said on Tuesday.

Germany's foreign ministry also tweeted their condolences following the attack: "We condemn the brutal attack in Istanbul and mourn with Turkey and with the dead. Our thoughts are with the relatives of the victims," the ministry said on Twitter.

John Bass, the US ambassador to Turkey, also commented on the tragedy.

The White House condemned the attack, saying the US stood together with its "NATO ally and valued partner."

"This horrific act is only the most recent of many terrorist attacks against Turkey," National Security Council spokesman Mark Stroh said in a statement.

Turkey has experienced a string of terrorist attacks since the beginning of the year. Kurdish militants claimed two separate bomb blasts in Ankara that claimed dozens of lives. And last month, at least eight people were wounded when a car bomb detonated in Istanbul, an attack that was also claimed by Kurdish extremists.

rs/bc/jm (AFP, AP, Reuters)