Gunman in Brazil holds at least 16 bus passengers hostage

A gunman is holding at least 16 bus passengers hostage in Rio de Janeiro on Tuesday, officials said, as police negotiate with the hijacker to try to end the standoff.

At least six people -- four women and two men -- have been freed so far from the vehicle that is stopped on heavily transited bridge connecting Rio with neighboring city Niteroi.

Heavily armed police including military and snipers have surrounded the bus as they negotiate with the gunman, who G1 news reported is armed with a gun, a tazer and gasoline.

A live broadcast of the scene shows five ambulances parked near the bus, receiving hostages as they are released.

A series of gunshots have been heard, Globo TV reported.

The gunman reportedly boarded the Rio-bound bus at 5:30 am (0830 GMT) and began threatening passengers.

Several lanes of traffic on the busy Rio-Niteroi bridge spanning Guanabara Bay have been paralysed during the ongoing hostage situation.

Commuters stopped on the bridge can be seen standing outside their cars watching the hostage situation and taking photos with their smartphones.

Rio state Governor Wilson Witzel tweeted that he was following the situation and was in direct contact with the military police commander.

"The absolute priority is the protection of hostages," Witzel said.

The gunman -- a young man wearing a white T-shirt and dark-colored trousers -- poked his head out of the bus at one point, showing his face, G1 reported.

It is not clear if the hijacker has made any demands or what his motivation was for taking the hostages.

Police special forces are negotiating with the hijacker.

"We ask the population to stay calm. Our police are at the scene," military police tweeted.

This is not the first time a gunman has hijacked a public transit bus in Rio.

In 2000, a gunman stormed a passenger bus in a fashionable neighborhood of the city. The hours long hijacking of bus 174 was later turned into an award-winning documentary.

The hijacker and one hostage were killed.