Turkey’s military has said it has seized power, but the prime minister said the attempted coup would be put down.
Footage on local television channels showed military vehicles blocking bridges over the Bosphorus in Istanbul and tanks deployed at the city’s main airport.
In Ankara, warplanes and helicopters roared overhead. There were also reports of gunshots.
An EU source monitoring events said the military coup attempt under way looked substantial and was “not just a few colonels”.
They said the military has control of airports and strategic points in Istanbul.
Turkish state broadcaster TRT said airports are closed and a curfew declared across the country.
An announcer on TRT said a new constitution would to be prepared as soon as possible.
A Turkish official, who did not want to be named, said soldiers had been deployed in other cities in Turkey, but did not specify which ones.
Dogan News Agency reported the national police directorate had summoned all police to duty in Ankara.
Turkish President Tayyip Erdogan is safe, a presidential source said, adding that a statement made on behalf of the armed forces announcing a takeover of the government was not authorised by the military command.
The source also urged the world to “stand in solidarity” with the Turkish people.
Prime Minister Binali Yildirim said the elected government remained in office.
The armed forces had taken power in the country to protect the democratic order and to maintain human rights, the military said in a statement sent by email and reported on Turkish TV channels.
All of Turkey’s existing foreign relations would be maintained and the rule of law would remain the priority, it said.
The state-run Anadolu news agency said the chief of Turkey’s military staff was among people taken “hostage” in the capital Ankara.
CNN Turk also reported that hostages were being held at the military headquarters.
Turkey, a NATO member with the second biggest military in the Western alliance, is one of the most important allies of the United States in the fight against the so-called Islamic State.
It is a principal backer of opponents of Syrian President Bashar al-Assad in that country’s civil war, and host to 2m Syrian refugees.
Mr Yildirim said a group within Turkey’s military had attempted to overthrow the government and security forces have been calledin to “do what is necessary”.
“Some people illegally undertook an illegal action outside of the chain of command,” Mr Yildirim said in comments broadcast by private channel NTV.
“The government elected by the people remains in charge. This government will only go when the people say so.”
Those behind the attempted coup would pay the highest price, he added.