Europol has announced that it busted a Macedonian heroin gang, responsible for smuggling huge quantities of the drug into the EU.

Police arrested 400 people and seized 100kg of heroin, cannabis and cocaine, as well as a large amount of cash, said the agency.

Four tons of drug cutting agents were also seized in the operation, with suspects arrested in Macedonia, Austria and Germany.

Operating from their bases in Germany, Austria and Macedonia, the gang smuggled narcotics into the EU along the so-called Balkan Route.

The joint police operation against the gang began in 2010, after the network rebuilt following the arrest of their leadership in a previous operation.

"We noticed that this organisation was so good that it was able to replace large quantities of high-quality heroin within days," Austrian security official Franz Lang told reporters in 2010.

The network of gangs operated according to a rigid structure, with members recruited in Macedonia given the prospect of earning good money in Austria and Germany.

Once recruited, members had very little contact with other people in the gang and had strictly defined roles, including mixing, packing and transporting high-quality heroin into the EU, where it was sold at low prices.

"The organisation was characterised by its strict division of labour and well-organised procedures," said Europol in a statement.

Most of the heroin in Europe originates in Afghanistan. It is then smuggled over the border into Iran, and shipped into the Balkans, from where it is transported into western Europe.