By Nick Tattersall and Ece Toksabay
ISTANBUL/ANKARA (Reuters) - Turkish troops said on Friday they had seized power but President Tayyip Erdogan vowed that the attempted coup would be put down and crowds answered his call to defy a curfew order and take to the streets to support him.
Gunfire and explosions rocked both the main city Istanbul and capital Ankara in a chaotic night, but by the early hours of Saturday there were indications that the coup was crumbling.
If successful, the overthrow of Erdogan, who has ruled Turkey since 2003, would mark one of the biggest shifts in the Middle East in years, transforming one of the most important U.S. allies while war rages on its border. If it fails, the coup attempt could still destabilize a pivotal country in the region.
"We will overcome this," Erdogan said, speaking on a video call to a mobile phone held up to the camera by an announcer on the Turkish sister station of CNN. He called on his followers to take to the streets to defend his government and said the coup plotters would pay a heavy price.
An official said Erdogan was speaking from Marmaris on the Turkish coast where he was on holiday. A Turkish official later said Erdogan's plane had landed in Istanbul.
A Turkish military commander said fighter jets had shot down a helicopter used by the coup plotters over Ankara. State-run Anadolu news agency said 17 police were killed at special forces headquarters there.
As the night wore on, crowds appeared to be answering Erdogan's call to take to the streets, defying orders by the coup leaders to stay indoors.
"We have a prime minister, we have a chief of command, we're not going to leave this country to degenerates," shouted one man, as groups of government supporters climbed onto a tank near Istanbul's Ataturk airport.
Prime Minister Binali Yildirim and other senior officials said the elected government remained in office. Yildirim and other officials blamed loyalists of a U.S.-based cleric for the coup attempt; his movement denied any part in it.
The United States declared its backing for Erdogan's government. Secretary of State John Kerry said he phoned the Turkish foreign minister and emphasized "absolute support for Turkey's democratically elected, civilian government and democratic institutions".
Crowds of people, some waving Turkish flags, gathered in major squares in Istanbul and Ankara to show support for the elected government. Police urged people to leave Istanbul's Taksim square, warning military aircraft could open fire.
Warplanes and helicopters roared over Ankara and Reuters reporters saw a helicopter open fire. Anadolu said military helicopters had fired on the headquarters of the intelligence agency.
Reuters journalists saw tanks open fire near the parliament building in Ankara, which they had surrounded. Anadolu later said a bomb hit the building. Smoke rose up from nearby, Reuters witnesses said. An opposition MP told Reuters parliament was hit three times and that people had been wounded.
In the first hours of the coup attempt, airports were shut, access to internet social media sites was cut off, and troops sealed off the two bridges over the Bosphorus in Istanbul, one of which was still lit up in red, white and blue in solidarity with victims of the truck attack in France the previous day.
Soldiers took control of TRT state television, which announced a countrywide curfew and martial law. An announcer read a statement on the orders of the military that accused the government of eroding the democratic and secular rule of law. The country would be run by a "peace council" that would ensure the safety of the population, the statement said.
Shortly afterwards, TRT went off the air. It resumed broadcasting in the early hours of Saturday.
Anadolu said the chief of Turkey's military staff was among people taken "hostage" in the capital Ankara, but Yildirim later said he was back in control. CNN Turk also reported that hostages were being held at the military headquarters.
"NOT A TINPOT COUP"
Early in the evening 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."
One European diplomat was dining with the Turkish ambassador to a European capital when guests were interrupted by the pinging of urgent news on their mobile phones.
"This is clearly not some tinpot little coup. The Turkish ambassador was clearly shocked and is taking it very seriously," the diplomat told Reuters as the dinner party broke up. "However it looks in the morning, this will have massive implications for Turkey. This has not come out of nowhere."
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, which seized swathes of neighboring Iraq and Syria.
The Pentagon said there was no impact on operations against Islamic State from the U.S. air base at Incirlik in Turkey.
Turkey is also one of the main backers of opponents of Syrian President Bashar al-Assad in that country's civil war, host to 2.7 million Syrian refugees and launchpad last year for the biggest influx of migrants to Europe since World War Two.
Celebratory gunfire erupted in Syria's capital Damascus after the army claimed to have toppled Erdogan. People took the streets to celebrate there and in other government-held cities.
Turkey 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.
Turkish officials blamed the attempted coup on followers of Fethullah Gulen, an influential cleric in self-imposed exile in the United States who once supported Erdogan but became a nemesis. The pro-Gulen Alliance for Shared Values said it condemned any military intervention in domestic politics.
After serving as prime minister from 2003, Erdogan was elected president in 2014 with plans to alter the constitution to give the previously ceremonial presidency far greater executive powers.
Turkey has enjoyed an economic boom during his time in office and has dramatically expanded its influence across the region. But opponents say his rule has become increasingly authoritarian.
His AK Party, with roots in Islamism, has long had a strained relationship with the military and nationalists in a state that was founded on secularist principles after World War One. The military has a history of mounting coups to defend secularism, but has not seized power directly since 1980.
Prime Minister Yildirim said a group within Turkey's military had attempted to overthrow the government and security forces have been called in to "do what is necessary".
"Some people illegally undertook an illegal action outside of the chain of command," 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."
(Reporting by Humeyra Pamuk, Ayla Jean Yackley, Nick Tattersall, David Dolan, Akin Aytekin, Tulay Karadeniz, Can Sezer, Gulsen Solaker, Ece Toksabay, Murad Sezer, Ercan Gurses, Nevzat Devranoglu, Dasha Afanasieva, Birsen Altayli and Orhan Coskun; Writing by Peter Graff; Editing by Catherine Evans)