The coup leaders have imposed a night-time curfew across the West African state, and have ordered the closure of land and air borders.
Presidential guard officers in Burkina Faso have seized power in a coup, with reports of more than 10 deaths amid protests in the capital, Ouagadougou.
A close ally of former President Blaise Compaore has been named the country's new leader, state television reports.
The US, France and African Union (AU) condemned the coup in the former French colony.
Those killed were shot dead by presidential guard forces in the capital, a civil society group said.
The claim by the influential Balai Citoyen group could not be independently verified.
Other reports said protesters had been assaulted and detained.
The coup leaders have imposed a night-time curfew across the West African state, and have ordered the closure of land and air borders, AFP news agency reports.
The headquarters of Mr Compaore's Congress for Democracy and Progress (CDP) party were ransacked in Ouagadougou as news of the coup spread, it adds.
The AU and regional body Ecowas, the Economic Community of West African States, called for the immediate release of "hostages", referring to interim President Michel Kafando and Prime Minister Isaac Zida, who were detained at a cabinet meeting in the president palace on Wednesday.
Their transitional authority was due to hand power to a new government after elections on 11 October.
Mr Compaore was ousted in a popular uprising last year after 27 years in power, and is currently in exile.
He was accused of committing widespread abuses, and trying to change the constitution to extend his term in office.
Some of his key allies had been barred from contesting the election.
A statement issued by the coup leaders said the West African state would be led by Gen Gilbert Diendere, Mr Compaore's former chief-of-staff.
In media interviews, he said he had no contact with Mr Compaore and would do everything to "avoid violence that could plunge the country into chaos".
An earlier announcement on state television said wide-ranging talks would be held to form a new interim government that would organise "peaceful and inclusive elections".
Transitional parliamentary speaker Cheriff Sy called for people to "immediately rise up" against the coup, and declared himself the leader of the West African state.
"We are in a resistance situation against adversity," Mr Sy added.
The US State Department said it was deeply concerned about the events unfolding in Burkina Faso.
French President Francois Hollande said he strongly condemned the coup "because an electoral process was under way".
However, France would not intervene militarily, he said.
The US said it was deeply concerned about events in Burkina Faso, and it condemned the seizure of power through unconstitutional means.
Burkina Faso is a key ally of the US and France in the campaign against militant Islamists in the region.