Russia says thwarted smuggling of giant meteorite to UK
The huge specimen weighing more than 2.5 tonnes is believed to have come from the Aletai meteorite, one of the largest known iron meteorites on Earth, it said.
Prosecutors have opened a criminal investigation.
"The strategically important cargo was discovered during checks on a sea container at the port of Saint Petersburg," the customs service said in a statement.
"When attempting to export it, it was declared as a garden sculpture. But a detailed inspection revealed that the origin and value of the cargo differed from the information declared," it added.
Video showed customs officers prying open a crate to find the rock, its surface grey and rugged.
The fragment could be worth approximately 323 million rubles ($4.2 million), the statement said.
The statement did not say who attempted to import the fragment, only that it was destined for the United Kingdom.
Scientists have expressed ethical concerns about the sale of meteorites, which are often coveted for research purposes and hold important clues about the make-up of the early solar system.
The Aletai meteorite was discovered in western China in 1898 and is thought to be at least 4.5 billion years old.