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Mark Cameron
The roof of a grain store has been partially destroyed in a "massive explosion" at Tilbury Port in Essex.
It happened at about 09:55 BST, the fire service said.
A witness described hearing the "horrendous" noise "like a bomb going off", while another said "flames shot up about 75m above the silos".
A Port of Tilbury spokeswoman confirmed a "major incident" had taken place. One person has been treated for smoke inhalation, the ambulance service said.
Tony Coughlin, who lives close to the port, said: "I saw it explode, it was just horrendous. It sounded like a bomb going off and I saw the flames shoot up into the air.
"About six of the silos exploded - the tops are all gone."
Mark Cameron, who was on board a ship in the docks at the time, tweeted: "There's just been an explosion in Tilbury harbour at the grain terminal - lifted the roof off."
He said: "I just felt the ship shake so came up on deck, a colleague said the flame shot up about 75m (246ft) above the silos."
Essex Fire and Rescue Service confirmed four fire engines were sent to the grain terminal shortly before 10:00 BST.
The East of England Ambulance Service sent crews to the scene together with the Essex and Herts Air Ambulance.
"One patient was transported by land to Basildon and Thurrock Hospital with smoke inhalation," a spokesman said.
"We remain on the scene as medical back-up for the fire service."
A Port of Tilbury spokeswoman said: "We confirm that there has been a major incident at the grain terminal within The Port of Tilbury.
"There have been no injuries reported and the emergency services are in attendance."