Dutch police arrested 18 people Tuesday they accuse of running illegal cannabis farms following a series of raids across the Netherlands and Belgium.

`With this operation, police have struck a serious blow to organised crime which hides behind the front of cannabis plant farming in south Holland,` the Brabant regional force said in a statement.

While the restricted sale of cannabis is permitted in the Netherlands, it is illegal to grow cannabis plants. Licensed coffeeshops may sell the drug, although any one customer can buy just five grammes per day.

The main suspect, a 53-year-old man, was arrested in Kaatsheuvel, south Holland after an 18-month long investigation for possession, production and exporting of cannabis as well as fraud and money laundering.

As many as 36 raids had been carried out in the Netherlands until the cannabis plants were uncovered, stashed away in six houses and a number of hangars. Authorities were in the process of `estimating` the exact quantity seized, Brabant police spokesman Jeroen Steenmeijer told AFP.

`We have the impression that these plants were destined for export abroad,` he added.

Police investigated the south of Holland, the outskirts of Amsterdam and north Belgium, where five people have been detained.

Illegal cannabis farming is said to be worth about two billion euros (2.6 billion dollars) per year, Dutch police said.