This is the dramatic moment smoke filled a Metro carriage in Mexico, sending passengers fleeing in panic.

Footage captured by commuter Manuel Ruz shows a crowded train on Line 1 near Merced station on December 17, engulfed in thick grey smoke.

Travellers were visibly distressed, with some pressed against doors and windows, while others forced and bent open windows to escape.

Manuel said: 'My partner and I were on this train car. As we left La Merced, there was a flash along with a sound like an explosion from the first car.

'The entire train stopped, and smoke began filling the cars. Looking out the windows, we saw police running toward the first car. We were heading toward Pantitlan and started to panic because the doors wouldn't open, and the smoke kept spreading.

'The police kept saying the doors couldn't open because the emergency levers had been activated. After resetting all the levers, they said no one could leave, and the train would move again. Despite shouting, the train started moving.

'A few metres later, a stronger explosion occurred, and even more smoke poured out. Fortunately, the last car had not yet entered the tunnel, and passengers used the windows there to escape.'

Authorities later confirmed the smoke was caused by a short circuit, possibly triggered by a metallic object.

After the locomotive was evacuated and cleared, Line 1 service resumed normal operations. There were no immediate reports of injuries.