A remote control bomb wounded 10 police officers near the Istanbul headquarters of Turkey's ruling AK Party, Turkish police said.
There are wounded people, many ambulances are coming here after the explosion, a witness told Reuters.
It was a very strong explosion like an earthquake, another witness told CNN Turk television.
Sadi Dinleyici, spokesman of the Independent Industrialists and Businessmen's Association (MUSIAD), whose headquarters are next to the AK Party building, told CNN Turk:
We were afraid the bomb had hit our building because the blast was so loud, windows were broken and the wounded police officers were given first aid in our building.
Istanbul police chief Huseyin Capkin said the blast was caused by a remote-controlled bomb on a motorcycle.
Television footage showed a wrecked light commercial vehicle, and a witness told Dogan news agency it had exploded as a police vehicle passed by.
Television footage also showed a fire engine and police vehicles surrounding the area, and shattered glass from nearby buildings littering the street after the explosion in Istanbul's Sutluce district.
Kurdish separatist rebels, far left groups, far right groups and Islamist militants, including al Qaeda, have all carried out bomb attacks in Turkey in the past.
(Reporting by Seda Sezer and Ece Toksabay, editing by Tim Pearce)