There was no immediate responsibility claim but Turkey has witnessed an increase in violence linked to Kurdish rebels and Islamic State militants.

Speaking at the scene of the blast in Beyazit district, Istanbul Gov. Vasip Sahin said a bomb placed inside a car detonated as a police vehicle passed by. The dead included seven police officers and four civilians, he said. At least three of the wounded were in serious condition.

Sahin declined to comment on who may be behind the attack and authorities imposed a news blackout preventing media from reporting details of the probe, citing concerns over security and police and forensic efforts to investigate the attack.

Recent Istanbul attacks have targeted security forces and tourism sites, contributing to a sharp dip in tourism and taking a toll on the economy.

Tuesday's bomb went off in a bustling Istanbul neighborhood just north of the iconic Golden Horn, where the Bosphorus Strait meets the Sea of Marmara. The area is home to the offices of provincial authorities, three universities and ancient sites including Roman-era aqueducts.

The police bus was flipped over by the force of the blast, which also damaged nearby buildings, among them a closed hotel whose entrance appeared gutted and windows were blown out. The blast also shattered the stained glass windows of a famous 16th-century Ottoman mosque, Sehzadebasi.

It wrecked several cars and forced the cancellation of some exams at nearby IstanbulUniversity.

President Recep Tayyip Erdogan visited some of the wounded at Istanbul's Haseki hospital, where two people were undergoing surgery.

"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 fearlessly and tirelessly until the end," he told reporters outside the hospital.

Hours after the explosion, police detained four suspects for their possible involvement in the attack, the state-run Anadolu Agency reported. The private Dogan news agency said the four had hired the car used in the bombing. The reports made no mention of the suspects' possible affiliation.

Prime Minister Binali Yildirim promised a full investigation.

"Our security and forensic units will carefully and diligently trace evidence and shed light on those who perpetrated this vile attack," he said.

Foreign Minister Mevlut Cavusoglu condemned the attack, which occurred on the second day of the holy Muslim month of Ramadan.

"They are cold-heartedly exploding bombs on a Ramadan day," Cavusoglu said

U.S. Ambassador John Bass condemned the "heinous attack," saying his country continues to "stand shoulder-to-shoulder with Turkey in the fight against terrorism."

The European Union expressed solidarity with Turkey as well as its commitment to "work closely to fight the global threat of terrorism."

"It's shocking that Turkey once again has to grieve for the dead and injured, who fell victim to a terrorist crime," German Foreign Minister Frank-Walter Steinmeier said.

NATO Secretary-General Jens Stoltenberg "firmly" condemned the attack and expressed solidarity with the victims and the people of Turkey, "struck once again by the scourge of terrorism."

NATO member Turkey is facing a broad array of security threats.

Rebels of the Kurdistan Workers' Party, or PKK, have targeted police and military personnel since July, when a fragile peace process between the rebels and the government collapsed. The Turkish military says it has killed 1,000 PKK militants since mid-March in the southeastern towns of Sirnak and Nusaybin.

A PKK offshoot has claimed recent attacks in Turkey as "revenge" for events in the southeast.

The Islamic State group has also been blamed for a series of deadly bombings in Turkey, which is part of the U.S.-led coalition against IS. The Turkish army has recently upped its shelling of IS targets in Syria in response to cross-border rocket fire from the group.

To date, IS hasn't claimed attacks on Turkish soil but has made threats against the country. (dmr)

___

Suzan Fraser reported from Ankara, and Mehmet Guzel in Istanbul also contributed.