MELBOURNE, March 23 (Reuters) - Three people were killed on Friday in a multiple car and truck crash which caused an explosion and fire inside a major commuter tunnel in Australia's second biggest city Melbourne.

As thick black smoke filled the Burnley Tunnel and rose out of ventilation funnels, hundreds of commuters abandoned vehicles and walked out of the tunnel under the Yarra River.

Metropolitan Fire Brigade said three people were killed in the collision between four trucks and three cars deep inside the 3.4 km (2 mile) long tunnel.

"From the people who are down there, the firefighters did confirm that there was three (dead)," Metropolitan Fire Brigade spokesman David Mann told reporters

Truck driver Matt Galea said he saw flames reach the roof of the tunnel and felt shock waves from the explosions.

"Being in the cabin I was a bit higher than other motorists and could see over the vehicles in front and just saw these huge flames hit the roof," Galea told Australian Associated Press.

"The next thing I knew was the emergency system instructing all drivers to stop their cars, turn off their ignitions, and leave their cars. We were all evacuated straight back out of the tunnel from the same entrance."

CityLink road spokesman Craig Little said the tunnel ventilation system appeared to work well and no-one seemed to be seriously affected by smoke or fumes.

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