The Turkish military has released a statement saying armed forces have "fully seized control" of running the country.
A group within the military had earlier on Friday engaged in what appeared to be an attempted coup, Turkey's prime minister said.
Both bridges in Istanbul have been blocked by Turkish military. Reason unknown. pic.twitter.com/HxyQn05Ox6
-- Gilgo (@agirecudi) July 15, 2016
Binali Yildirim told NTV television: "It is correct that there was an attempt."
Yildirim did not provide details, but said Turkey would never allow any "initiative that would interrupt democracy".
But the statement attributed to the military later said the takeover had been successful.
-- Mark Lowen (@marklowen) July 15, 2016
Earlier, military jets were heard flying over the capital, Ankara.
Media reports said ambulances were seen in front of Turkey's military headquarters.
"We are focusing on the possibility of an attempt (coup)," Yildirim had said.
"There was an illegal act by a group within the military that was acting out of the chain of military command. Our people should know that we will not allow any activity that would harm democracy."
The Dogan news agency says traffic on the Bosphorus and Fatih Sultan Mehmet bridges was blocked.
Video footage and photos showed the bridge being blocked by military vehicles.