If successful, the overthrow of President Tayyip Erdogan, who has ruled Turkey since 2003, would amount to one of the biggest shifts in power in the Middle East in years.

Meanwhile, people have taken to the streets amid a sense of unreality.

Speaking via Skype to a TV station,  Erdogan called the nation to back him.

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 Islamic State.

It is a principal backer of opponents of Syrian President Bashar al-Assad in that country's civil war, and host to 2 million Syrian refugees.

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 the capital Ankara, warplanes and helicopters roared overhead. A Reuters journalist heard gunshots.

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.

Martial law has now been imposed. 

Turkish President Tayyip Erdogan is safe, a presidential source said.

U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, speaking jointly after talks in Moscow, both said they hoped bloodshed would be avoided.

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 Islamic State.

It is a principal backer of opponents of Syrian President Bashar al-Assad in that country's civil war, and host to 2 million Syrian refugees.

The country has been at war with Kurdish separatists, and has suffered numerous bombing and shooting attacks this year, including an attack two weeks ago by Islamists at Istanbul's main airport that killed more than 40 people.

A senior EU source monitoring the situation said: "It looks like a relatively well orchestrated coup by a significant body of the military, not just a few colonels. They've got control of the airports and are expecting control over the TV station imminently. They control several strategic points in Istanbul.

"Given the scale of the operation, it is difficult to imagine they will stop short of prevailing. It's not just a few colonels," the source repeated.