Shots have been heard after an armed man hijacked a bus and took 37 passengers hostage in Rio de Janeiro, according to reports.
The bus is blocking all lanes on the Rio–Niterói Bridge in the Brazilian city as a major police operation is launched.
Police have been trying to negotiate with the 'hijacker' as they surround the vehicle, local media reports.
Globo News, citing witnesses and police sources, reported that shots have been fired.
The masked man, believed to have petrol with him, was threatening to burn the bus from the inside, ABC.es reports.
Six hostages have since been released, officials have confirmed.
Authorities are trying to bring the incident to a peaceful conclusion, CNN reports.
The bridge is currently closed in both directions.
The incident began at 5.30am local time (8.30 GMT), reports say.
Police have told reporters the incident appears to be 'premeditated'.
The hijacker released some of the hostages separately and at time intervals, reports say.
"We have a man who has identified himself as a military policeman," Sheila Sena of the Federal Highway Police told globo.com.
"He is threatening to throw gas on the bus, putting passengers in danger.
"We are in negotiations with him to release more hostages, we do not know what his real purpose is."
We'll be bringing you the very latest updates, pictures and video on this breaking news story.
