Twin blasts outside a football stadium in Istanbul killed at least 29 people and injured 166, Turkey's interior minister Suleyman Soylu said.
The two explosions, believed to be a car bomb and a suicide bomb, exploded at about 10.30pm on Saturday outside the newly-constructed Vodafone Arena Stadium, also known as Besiktas Stadium, in the European side of the city.
Minister Soylu said arrests had been made, but more details were not given.
President Recep Tayyip Erdogan released a statement saying the deadly attack targeted police and civilians, and aimed to maximize casualties. He did not confirm the death toll.
"A terrorist attack has been carried out against our security forces and our citizens," he wrote.
"As a result of these attacks unfortunately we have martyrs and wounded."
A video uploaded to YouTube purported to show the blast from the eastern bank of the Bosphorus.
Turkish journalist Ezgi Basaran tweeted a picture of debris from the scene.
"It is thought to be a car bomb at a point where our special forces police were located, right after the match at the exit where Bursaspor fans exited, after the fans had left," Turkey's state-run Anadolu Agency quoted Soylu as saying.
He later told reporters in Istanbul the first, larger explosion happened on a hill overlooking the stadium and the second struck Macka Park.
"It was like hell. The flames went all the way up to the sky. I was drinking tea at the cafe next to the mosque," Omer Yilmaz, who works at the neighboring Dolmabahce mosque, told Reuters.
"People ducked under the tables, women began crying. Football fans drinking tea at the cafe sought shelter, it was horrible."
Turkey's privately-owned NTV channel said first bomb targeted a bus for riot police. It reported 38 people were wounded.
Witnesses said they heard gunfire after the explosions.
Local TV stations showed more than a dozen ambulances at the stadium and a police helicopter flying overhead with its searchlights on.
The Besiktas sports club, who were playing visiting team Bursaspor at the stadium earlier on Saturday, "strongly condemned" terrorism and the attack in a statement.
Bursaspor said none of its fans were wounded and wished "a speedy recovery to our wounded citizens."
Turkish media authorities had partially banned outlets from covering the attacks, citing national security concerns.
It said "to avoid broadcasts that can result in public fear, panic or chaos, or that will serve the aims of terrorist organizations."
Media blackouts are common after such attacks in Turkey.
No groups claimed responsibility for the blasts.
Turkey had a year of militant attacks in its two biggest cities that left dozens dead and put the country on high alert. Kurdish militants twice struck in Ankara, and suspected Islamic State group suicide bombers hit Istanbul on three occasions.
Erdogan's ruling AKP party today lodged a constitutional reform bill that would expand his powers and that critics said would weaken democratic checks and balances.