Gunfire and explosions have been reported after the army group claimed martial law
The Turkish prime minister has said the situation is "largely under control" following a military coup attempt.
An army group earlier claimed to have "fully seized control" of the country amid reports that at least one explosion has hit parliament in Ankara.
Recep Tayyip Erdogan has now landed back in Istanbul and has addressed the country.
He said the attempted coup was 'an act of treason' and those responsible will pay a 'heavy price' for their involvement.
Video footage showed the president among a crowd of cheering supporters outside the airport.
An Istanbul hospital said it is treating at least 150 wounded people.
The prime minister has said 120 people have been arrested, according to AFP news agency.
However, amid the reports of the situation being under control, the offices of broadcaster CNN Turk were apparently taken over by Turkish military.
They appeared to begin broadcasting again in the early hours of Saturday morning.
Further explosions have reportedly occurred at Turkish parliament, as well as Ataturk airport and Taksim Square in Istanbul.
UPDATE Explosions rock Turkish parliament, smoke seen,helicopter hits the GA room.PM orders jets from Eskişehir base to destroy the chopper.— CNN Türk ENG (@CNNTURK_ENG) July 16, 2016
Earlier, an announcer on Turkey's state broadcaster TRT read out a live statement, reportedly on the orders of the military, announcing that the army had taken control.
The statement said the country was being run by a "peace council", declaring that martial law and a nationwide curfew were in place.
TRT had been off air for several hours, but is now broadcasting again. Staff say they were taken hostage.
Turkey's leaders have confirmed that a coup "attempt" has taken place, but denied that the military is in control.
Prime Minister Binali Yildirim told the NTV broadcaster a no-fly zone has been declared over Ankara, insisting the "situation is largely under control".
Numerous reports of aerial attacks, gunfire and explosions are emerging from Istanbul and the Turkish capital Ankara.
An unconfirmed report from the state-run Anadolu news agency suggests that parliament in Ankara has been hit by at least one bomb.
Lawmakers are currently hiding in shelters inside the building, one member of parliament told news agency Reuters.
Footage shown by the NTV station appeared to show an explosion in the city:
It has also been reported by news agencies that 17 police officers have been killed in a helicopter attack:
BREAKING: 17 Turkish officers reportedly killed in helicopter attack on police special forces headquarters on outskirts of Ankara.— The Associated Press (@AP) July 15, 2016
NTV has reported that a Turkish fighter jet shot down a military helicopter used by coup plotters over Ankara, according to Reuters and AFP.
In Istanbul, meanwhile, TV footage shows people ducking and running for cover as shots ring out on the city's main Bosporus bridge.
According to a report quoted by news agency AP, soldiers fired the shots at a crowd of people trying to cross the bridge in protest against the coup, injuring several.
Military vehicles earlier blocked two main bridges in the city, including the Bosporus bridge.
Gunfire has been heard in several areas of Istanbul, including the Ataturk airport, amid reports that all flights have been cancelled.
The Turkish government has reported protesters blocked forces attempting the coup on some streets.
The European Union has called for "restraint and respect for democratic institutions" in the country.
Several world leaders, including UN Secretary General Ban Ki-moon, have issued statements calling for calm.
President Obama has spoken to Secretary of State John Kerry, who is currently in Moscow.
The White House says they agreed all parties in Turkey should support the democratically-elected government Turkey and avoid violence.
The Department of Foreign Affairs here is advising Irish citizens to avoid non-essential travel to Turkey.
"The Department of Foreign Affairs and our embassy in Ankara are monitoring the evolving situation in Ankara and Istanbul where a heightened security presence and some incidents of violence are reported," it said in a statement.
We are aware of situation in Turkey. See updated travel advice here: https://t.co/z11g3pt4Bn. Call 01 408 2000 if you have any concerns— IrishForeignMinistry (@dfatirl) July 15, 2016
In a bizarre interview with a Turkish broadcaster, President Erdogan called on people to take to the streets.
Speaking from an unknown location on a mobile phone held up by the presenter, he said the uprising attempt was being run by a minority within the military, warning that it would be given the "necessary response."
A Turkish state news agency reported that a top general was among several people being "held hostage" at the military headquarters in Ankara.
It followed media reports that ambulances had been seen in front of the building.
NTV has shown footage of tanks at the entrance of Ataturk airport.
Photos showed tanks blocking two bridges in Istanbul, while military jets were heard flying over Ankara.
In a statement, Twitter said it had "no reason to think we’ve been fully blocked in Turkey, but we suspect there is an intentional slowing of our traffic in country."
According to Turkish media, the military statement said the army had taken action to: "reinstall the constitutional order, democracy, human rights and freedoms, to ensure that the rule of law once again reigns in the country, for the law and order to be reinstated."
The military statement went on to say that "all international agreements and commitments will remain. We pledge that good relations with all world countries will continue."