Belgian police seized 11.5 tonnes of pure cocaine hidden in a scrap metal container in the country's single biggest-ever haul, prosecutors said on Thursday.

The find was made October 27 in the port of Antwerp when five scrap metal containers from South America were given a thorough inspection, a statement said.

The haul had an estimated value of 450 million euros ($525 million) in its pure form, it said. Once cut with other products and sold to consumers, the street value would have been double that. 

Antwerp is Europe's second largest goods port after Rotterdam, but is the number one gateway for cocaine to the continent, thanks to its close trade links with South America.

Investigators said the containers initially landed in the Belgian port of Zeebrugge and were then transferred by barge to the port of Antwerp.

The cocaine was concealed in a big steel container that was itself fitted inside a larger freight container which had some scrap metal parts thrown in as an attempt to disguise the load.

It was shipped from Guyana. The final destination was a company located just across the Dutch border, they said.

After searches in Belgium and the Netherlands, three people were arrested, two in Belgium and one in the Netherlands.

The case is linked to a sting operation in September and October from which 22 suspects are still being held and three million euros in cash was seized.

Among those arrested were a former head of the gendarmerie and three active police officers.

The city of Antwerp and the neighbouring province of Limburg have been struck by a crime wave connected to the drug trade, including hostage-taking, grenade attacks and murders.

Apart from cocaine, Limburg is also known to host secret laboratories producing synthetic drugs for gangs mainly based across the border in the Netherlands.

- Belgian record haul -
While a record haul for Belgium, the amount of cocaine seized trails ones made in other jurisdictions in recent years.

In July last year, for instance, the US Coast Guard recovered a total of 13 tonnes of cocaine left floating in international waters between Central America and the United States, dumped by a go-fast boat surprised by authorities.

Ecuador in 2016 grabbed 12 tonnes of cocaine hidden in sacks of salt meant to be shipped to Belgium.

In Nigeria in 2006, 14.2 tonnes of cocaine hidden in a container labelled as carrying cement were discovered in Lagos's port. 

In 2007, off Panama's Atlantic coast, the US Coast Guard found 19 tonnes of cocaine on a Panamanian-flagged vessel.

But perhaps the world record goes to a haul in Colombia that same year, in 2007, when the country's military seized between 20 and 25 tonnes of cocaine off its Atlantic coast.

According to the UN Office on Drugs and Crime, the United States remains the biggest market of cocaine users, with Europe coming second, and growing.

There is "evidence of an increase in the availability of cocaine of the highest reported purity in over a decade in the European Union," the UNODC said in its latest report this year, citing figures from Europol and the European Monitoring Centre for Drugs and Drug Addiction.

Cities in Western Europe showed large traces of cocaine in their sewage, UNODC said, "in particular in Belgium, the Netherlands, France, Spain, Switzerland and the United Kingdom".