A Burundi army general says President Pierre Nkurunziza has been "overthrown", amid unrest over his bid to be re-elected for a third term.
Major General Godefroid Niyombareh said a salvation committee had been set up, but his level of support is unclear.
Thousands protesting against the president in the capital, Bujumbura, are now reported to be celebrating.
President Nkurunziza is currently in Tanzania. The presidency's Twitter site said the coup attempt had failed.
Mr Nkurunziza has been meeting other East African leaders to discuss the crisis. His aide dismissed the coup claims as "a joke".
In a series of Twitter postings, the Burundi presidency at first said the situation was under control and that there was no coup, then that the attempted coup had failed.
The BBC's Maud Jullien in Bujumbura says there is heavy gunfire as protesters escorted by the army head to the city centre.
She says at least three people were killed in the capital's Kabondo district but the circumstances of their deaths remain unclear.
Soldiers loyal to the president have been firing to protect the offices of the national broadcaster in Bujumbura.
Police were reported to have vanished from the streets.
Gen Niyombareh, in a statement read to reporters in a military base, said he did not recognise the leadership because the president's bid for a third term
In a radio broadcast on a private station, Gen Niyombareh said: "The masses have decided to take into their own hands the destiny of the nation to remedy this unconstitutional environment into which Burundi has been plunged.
"The masses vigorously and tenaciously reject President Nkurunziza's third-term mandate... President Pierre Nkurunziza has been relieved of his duties. The government is overthrown."
Gen Niyombareh's "national salvation committee" comprises at least five other army and police generals.
He said in the radio broadcast that the mission of the body was the "restoration of national unity... and the resumption of the electoral process in a peaceful and fair environment".
The unrest began on 26 April and has led to the deaths of more than 20 people.
Tens of thousands of Burundians have fled to neighbouring states in recent weeks.
President Nkurunziza has rejected calls to postpone next month's election.
The 51-year-old former rebel leader argues that he is entitled to run for a third term because he was first appointed to the role by parliament in 2005.
The constitution states a president should govern only for two terms, but earlier this month a court upheld Mr Nkurunziza's interpretation.