Two explosions have hit the Turkish city of Istanbul, killing at least 38 people, mostly police personnel, and wounding more than 160 others, according to authorities.
Suleyman Soylu, the interior minister, announced that 30 police officers, seven civilians and an unidentified person were killed. So far, 13 suspects have been detained in connection with the attack, Soylu said.
Health Minister Recep Akdag said 20 people were discharged from hospitals while six of the wounded were in intensive care, according to Turkish national daily Hurriyet.
Three of the wounded are still in a critical condition, he said.
We have witnessed, once more here in Istanbul, the ugly face of terror which tramples down any form of value and morals
"I heard two loud explosions within 30 seconds of each other," Omer Yilmaz, a cleaner at the nearby Dolmabahce Mosque, told Al Jazeera.
"I saw flames. I walked towards the police bus which was on fire and I saw policemen inside."
Turkey declared one day of mourning after the twin blasts ripped through the heart of Istanbul, the state-run Anadolu news agency said on Sunday.
Prime Minister Binali Yildirim also ordered flags to fly at half-staff, Anadolu said.
Turkish President Recep Tayyip Erdogan said the blasts were timed to cause maximum loss of life.
"A 'terrorist' attack has been carried out against our security forces and our citizens," he said.
"We have witnessed, once more here in Istanbul, the ugly face of 'terror' which tramples down any form of value and morals."
Erdogan postponed a scheduled visit to Kazakhstan for talks with his counterpart Nursultan Nazarbayev.
Police evacuated the area around the Besiktas football arena following the explosions, which the interior ministry said was a car bomb attack. The second explosion at a nearby park was reportedly carried out by a suicide bomber.
The arena is located across the historic Dolmabahce Palace overlooking the Bosphorus Strait.
Al Jazeera's Sinem Koseoglu, reporting from Istanbul, said all the roads leading to the arena were closed to the public. Public transportation was also suspended in the wake of the blasts, which occurred about two hours after a football match attended by thousands of people at the Besiktas Vodafone arena.
A video posted on Twitter captured the moment of the explosion.
Vodafone Arena ( Maçka Parkı ) etrafinda gerceklesen #patlama dan görüntüler.
The Turkish broadcaster NTV said the blasts targeted police vehicles leaving the arena after fans had already dispersed following the football match.
There was no immediate claim of responsibility for the attack. But Fadi Hakura, a security analyst, told Al Jazeera that the attack has all the hallmarks of a Kurdistan Workers' Party (PKK) operation.
"PKK tends to target any representations of the Turkish state, particularly security forces," he said.
"Sadly, this attack will further reinforce the impression that Turkey is becoming increasingly insecure and unstable, that Turkey is no longer the bastion of tranquillity in a very troubled region."
Turkey has experienced a bloody year of attacks in its two biggest cities that have left dozens dead and put the country on high alert.
Kurdish armed groups have twice struck in the capital Ankara, while suspected Islamic State of Iraq and the Levant (ISIL, also known as ISIS) suicide bombers have hit Istanbul on three occasions.
In June, 47 people were killed in a triple suicide bombing and gun attack at Istanbul's Ataturk airport, with authorities pointing the finger at ISIL.
Another 57 people, 34 of them children, were killed in August in a suicide attack by an ISIL-linked bomber at a Kurdish wedding in the southeastern city of Gaziantep.
The country is also still reeling from a failed July 15 coup blamed on the US-based preacher Fethullah Gulen that has been followed by a mass dismissal of his alleged supporters from state institutions.
The government imposed a temporary blackout on coverage of the explosions in Turkey, citing national security reasons.
It said security forces in Istanbul are on high alert.