Rio's famous carnival celebrations have officially kicked off, after Friday morning was marred by more street violence between security forces and drug gangs. One armed gang of about 30 men initiated a three-hour shoot-out with police on one of Rio's biggest thoroughfares, Brazilian media reported. A motorist died after being shot at point-blank range by one of the group when he refused to give up his car. Tens of thousands of armed police - and, for the first time, troops - have been sent onto the streets to help guard Rio de Janeiro's carnival following the wave of violence.