Update 9.05pm: It is being reported at least 13 people have been killed following an explosion in Istanbul.
There have also been unconfirmed reports of a second explosion and gunfire being heard.
Earlier: A bus carrying riot police was hit by a car bomb blast near a major football stadium in Istanbul, wounding at least 20 officers, authorities said.
Witnesses said police were deployed and had cordoned off the area as smoke rose from the newly built Besiktas Stadium.
They said gunfire could be heard in what appeared to have been an armed attack on police.
The private NTV channel said the target of the attack was a bus for riot police.
Turkey's Interior Minister Suleyman Soylu confirmed the casualty toll and said the wounded were police officers.
"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," Mr Soylu told Turkey's state-run Anadolu Agency.
"We have no information on the number of dead. God willing we hope there won't be any. The wounded are police."
Images broadcast on television showed more than a dozen ambulances on a street near the stadium and a police helicopter flying overhead with its search lights on.
Prime Minister Binali Yildirim and Istanbul Governor Vasip Sahin had been notified, the agency said.