The Foreign Office has advised British citizens in Turkey to avoid public places and be vigilant as tanks took to the streets of the capital Ankara in a military coup.
The whereabouts of President Recep Tayyip Erdogan were unclear, as military figures announced they had "fully seized control" of the country - a member of Nato and an applicant for EU membership - in a bid to restore democracy and the rule of law.
Prime minister Binali Yildirim insisted the development was no more than an "attempt" at a coup, as gunfire was heard outside military headquarters, fighter jets flew over the capital and military vehicles blocked key bridges and access to the airport.
"We are focusing on the possibility of an attempt," Mr Yildirim told NTV television.
"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."
News reports said Mr Erdogan was safe and would make a televised statement and later, speaking to CNN-Turk through FaceTime, he called the actions by the military "an attempt at an uprising by a minority within our armed forces".
He said "I don't believe this coup attempt will be successful. There is absolutely no chain of command here. Right now the chain of command has been put on hold."
In a statement, the military said it seized control "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 statement added: "all international agreements and commitments will remain. We pledge that good relations with all world countries will continue".
The Turkish news agency Dogan reported that soldiers opened fired on people trying to cross Istanbul's Bosporus bridge protesting against the attempted coup, and some had been wounded.
In London, the Foreign Office urged Britons in the country to monitor the travel advice it provides on its website.
"We are aware of the current developments in Turkey. We are urgently seeking more information," said a Foreign Office spokesman.
"Until the situation becomes clearer, we advise you to avoid public places and remain vigilant."
Shadow foreign secretary Emily Thornberry said: "We urge everyone in Turkey, in whatever position, to show calm and restraint in the coming days, and avoid any actions that would divide the country or damage its standing in the world.
"There is no excuse on any side for violence or abuse of human rights, and we would urge all parties to engage with the international community to preserve peace and democracy, and resolve this situation.
"In particular, we urge all those in a position of authority to reassure Britons and other foreign nationals currently visiting Turkey that they are safe, and will be allowed to return home without impediment at the earliest opportunity."
Former Liberal Democrat leader Lord (Menzies) Campbell told the Press Association: "It should come as little surprise that the army which sees itself as the proud guardian of the secular tradition in Turkey, has responded to President Erdogan in this way.
"His increasing steps to create a greater religious element in governance and his restrictions on civil rights have provoked severe discontent.
"Instability in Turkey has profound consequences for its relationship with the EU and the issues of immigrants and for Nato of which Turkey is a long-standing and military and politically significant member."
The Association of British Insurers said in a statement: "In light of the coup in Turkey, anyone booked to travel to the country should contact their tour operator, travel agent, or airline for advice. If alternative arrangements are made you should be able to transfer your travel insurance to the new destination.
"You should regularly check Foreign and Commonwealth Office travel advice. Anyone in Turkey should speak to their tour operator or airline for advice."