The military in Turkey said it has taken over the country, with soldiers at strategic points in Istanbul and jets flying low in the capital, Ankara.
A statement read on TV said a “peace council” now ran the country and there was a curfew and martial law.
It is unclear who the army group is. There are reports some senior army officials have been detained.
President Recep Tayyip Erdogan said he would overcome what he called an uprising by a minority.
He told CNN Turk by mobile phone the action was by a “parallel structure” that would bring the necessary response.PM Binali Yildirim had earlier denounced an “illegal action” by a military “group”, stressing it was not a coup. He said that the government remained in charge.
The military group’s statement on national broadcaster TRT, read by an announcer, said that democratic and secular rule of law had been eroded by the current government. There would be new constitution, it said.
A Turkish presidential source told Reuters news agency that the statement was not authorised by the army’s command.
There are reports Turkey’s top general, General Hulusi Akar, is among those taken hostage at the military HQ.
Mr Yildirim told NTV by telephone: “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.
“Those who are in this illegal act will pay the highest price.”
Traffic has been stopped from crossing both the Bosphorus and Fatih Sultan Mehmet bridges in Istanbul.
There are reports of gunshots in the capital Ankara. Other reports said soldiers were inside buildings of the Turkish state broadcaster in Ankara.
Gunfire was also heard outside Istanbul police HQ and tanks are said to be stationed outside Istanbul airport. All flights are cancelled, reports say.
One European Union source told Reuters that the military action “looks like a relatively well-orchestrated coup by a substantial body of the military, not just a few colonels”.
Visiting Moscow, US Secretary of State John Kerry said he hoped for peace and “continuity” in Turkey.