Sierra Leonean police say they have seized drugs from Ecuador, hidden in a shipping container full of nappies.
Police Assistant Inspector General Morrie Lengor said UK anti-drug officers have been called in to assist with investigations.
Latin American drug cartels use West Africa as a transit point to smuggle narcotics to Europe.
Five people - Sierra Leoneans and foreigners - have been arrested, Insp Gen Lengor said.
The arrests took place following a series of raids in the capital, Freetown.
A source at the police laboratory said initial tests showed that it was a cocaine haul, reports the BBC's Umaru Fofana from Freetown.
Insp Gen Lengor said he could not confirm the drugs were cocaine.
The UK's Serious Organised Crime Agency (Soca) would assist in conducting conclusive tests and carrying out further investigations, he said.
Our reporter says the haul appears to be big, as the container of nappies in which the drugs were hidden was 40 ft (12.19m) tall.
The container had arrived from Ecuador and, strangely, had duty free concessions, our reporter says.
He says the bust comes just weeks after a light aircraft mysteriously landed at the main international airport in Freetown.
The plane had no landing rights and police said they had made several arrests, our reporter says.
In 2008, Sierra Leonean police impounded a plane loaded with cocaine.
A key suspect, Gibril Kamara, a Sierra Leonean, was arrested in neighbouring Liberia last year, following investigations involving US authorities.
He was convicted in the US earlier this year on drug-trafficking charges.
In October, The Gambia sentenced eight foreigners to 50 years each in prison for trying to smuggle two tonnes of cocaine from Latin America to Europe.
The eight - four Venezuelans, two Dutch men, a Nigerian and a Mexican - were arrested last year in an operation involving Gambian and UK anti-drug enforcement agencies.
The West African coast has emerged as a busy route for drugs from Latin America to Europe, with cartels taking advantage of the region's poverty and weak border controls.