There have been reports of military helicopters shooting at targets in the Turkish capital Ankara, following a military "coup".
The news came shortly after a bizarre television appearance by President Recep Erdogan, who was interviewed on CNN via a Skype call on a smartphone. During the interview, he called for the population to resist - although he added he was still in control of the country.
Meanwhile, bridges in Istanbul appeared to have been blocked off, while reports suggested Ataturk Airport had also been blockaded by the military.
Barry Stocker, a British professor at Istanbul Technical University, has reported large crowds and chanting on the city's streets, along with ambulances and sirens. Tanks have been firing near parliament, Stocker said, while soldiers are occupying parts of Taksim, in the popular central district of Istanbul.
"A period of increasing AKP and Erdogan dominance of Turkush institutions appears to have come to an end as the Turkish army revives its tradition of launching coups," Stocker told City A.M.
Details coming out of Turkey were unclear: although the military was reported to have declared martial law, Prime Minister Binali Yildrim said the attempted coup had been unsuccessful.
Surreal. President Erodgan on skype on Tv presenter's phone on CNN Turk blaming "parallel state" for coup attempt pic.twitter.com/7u76kOwhxL— Laura Pitel (@laurapitel) July 15, 2016
The lira fell 4.6 per cent against the dollar to $0.33132, with social media suggesting people were queuing at cash machines.
In a tweet, newly-inaugurated foreign secretary Boris Johnson said he was "very concerned" - as the Foreign Office issued new travel advice to Britons in the country.
Meanwhile, flight tracker AirLive said flights to the country were being diverted, with only Turkish Airlines aircraft landing in Istanbul.