The ease with which would-be terrorists can purchase battlefield weapons on the streets of Europe was brought into the spotlight, as it emerged the station used by the gunman to launch his botched attack is surrounded by a market notorious for illegal weapons trade.

Ayoub el-Khazzani boarded the train he had chosen for the attempted massacre at Brussels' Gare du Midi, metres from where the terrorists behind the Charlie Hebdo atrocity in Paris this year bought their arsenal.

He was carrying a short-stocked Kalashnikov assault rifle, at least five magazines of ammunition, a handgun and knives. He has told police he had stumbled across the weapons "by chance" in a Brussels park, and planned to rob the passengers.

The Eurostar rail terminal is a short metro ride from the buildings of the European Union institutions, and on the day of the attempted attack it was teeming with tourists.

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But the station is surrounded by grimy streets that host a weekly market, notorious for selling weapons including Kalashnikov assault rifles for as little as 1000 ($1700).

Police are examining surveillance camera footage.

It is where the terrorists behind the murderous attacks on a Paris supermarket and Charlie Hebdo magazine in January are believed to have purchased weapons from Belgian arms dealers.

Amedy Coulibaly, whose attack on a kosher supermarket killed four Jewish Parisians, bought his Czech-built Scorpion machine gun and Russian Tokarev handgun in Brussels and Charleroi in Belgium. The Kouachi brothers, who killed 12 people at Charlie Hebdo, received their weapons from Coulibaly, who bought them near Gare du Midi for around 3800 ($8900).

The cases highlight the vast trade in illegal weapons across continental Europe.

Many weapons are left over from the wars in the Balkans.

Typically, a Kalashnikov automatic rifle can be bought for several hundred euros in Bosnia, then sold on in Belgium or the Netherlands for use in organised crime for 10 times that sum. Smugglers can move weapons between EU countries without customs checks.

Earlier this month police in Charleroi broke up a suspected international arms smuggling ring.

They were alleged to be using forged paperwork to import Glock and Sauer handguns fitted with silencers, Browning rifles and shotguns.

The supply of weapons designed for use on the battlefield has been expanded after the Russian military upgraded its standard-issue assault rifle, creating vast stockpiles of older weapons.

Those can filter from legitimate second-hand deals with poorer allies into the criminal underworld.

Gun ownership is permitted by licence in Belgium, although automatic and military-grade weapons are not.

There are around 900,000 firearms circulating in the country of 11 million, of which some 300,000 are thought to be unregistered.

In neighbouring France, estimates of the number of illegal weapons have risen sharply in recent years.

Law enforcement has shifted online.

Crypto-currencies such as Bitcoin, and the so-called dark web of encrypted websites accessed by specialist software, have allowed criminals to buy online relatively anonymously.