On the morning of Saturday, Nov. 1, 1755, the sky above the Portuguese capital "was perfectly serene and clear," a British merchant wrote in a letter home.

Then, at about 9:30 a.m., the table he was writing on at his lodgings began to tremble and he heard "a strange, frightful kind of noise underground, resembling the hollow, distant rumbling of thunder."

Next came the devastating first shock of a massive earthquake that left behind "a scene of utmost horror and desolation."

Lisbon, a major Atlantic trading port and the center of Portugal's intercontinental empire, was largely flattened, with thousands killed and tens of thousands injured in minutes.

The anonymous letter, published by the British Historical Society of Portugal in 1985, was apparently intended for a relative. It was unsigned and never sent.

It provides one of the few trusted descriptions of an event that invited exaggeration.

As his building collapsed around him, the dust dimmed the sky and almost choked him, he wrote.

When the air cleared, the British merchant helped a woman with a small child in her arms out of the building and into the street.

"At which instant there fell a vast stone from a tottering wall and crushed both her and the child in pieces."

He made his way along steep, narrow streets cluttered with rubble and mangled bodies.

"I could hardly take a step without treading on the dead or dying," he wrote.

The merchant recounts how he clambered over rubble to the Tagus riverbank, where ships were "tumbling and tossing about, as in a violent storm" because of the Earth's underwater movement.

Many others who had sought refuge by the river from falling buildings would die as the tsunami struck.

"In an instant there appeared at some small distance a vast body of water, rising as it were like a mountain, it came on foaming and roaring, and rushed towards the shore with such impetuosity that, though we all ran for our lives as fast as possible, many were swept away," the merchant wrote.

He, like most others who could walk, fled the city where fires burned for six days.

When he went back for his possessions, he was unable to recognize his street in the wreckage, and the stench of rotting corpses was everywhere.

"This extensive and opulent city is now nothing but a vast heap of ruins," he concluded. "The rich and poor are at present upon a level."

Thousands died on the shores of Lisbon's Tagus River when a tsunami followed the huge quake of Nov. 1, 1755.