Zeeshan locks his supermarket trolley to a tree like others would lock their bicycles: it is a crucial tool for him and other immigrants who comb Athens streets for scrap metal they can sell.

The strange contraptions appeared in the Greek capital about a year ago, and are now seen everywhere, from the city centre to far-flung suburbs as their conductors weave in and out of traffic.

It takes considerable skill to manoeuvre a supermarket caddy that is overflowing with tin cans, electronic appliances, bed-springs, oil cans, stoves, cables and radiators.

After a day going through rubbish bins, the balance becomes precarious while potholes or curbs pose hidden obstacles in an urban quest that evokes images from the post-apocalyptic road movie "Mad Max".

Tens of kilometers (miles) later, the moment of truth arrives, at a weigh station. Scrap metal dealers in the southern neighbourhood of Tavros are used to seeing exhausted collectors, many from Bangladesh, arrive.

"Greek or Albanian gypsies who collected abandoned trash in their trucks work less now because people have been throwing out less during the crisis" that has held Greece in its grip for several years, said Anna Darsinou.

Her family has been dealing in scrap metal for about three decades.

The business is evidence of the poverty in which thousands of illegal immigrants live in Greece, a gateway to the EU but a trap for those who do not manage to continue their journey to more prosperous member states.

"There is no other kind of work here," said Zeeshan, who arrived from Pakistan a year and a half ago on a trip that cost him 3,000 euros ($4,000).

The 26-year-old said he had studied computer design back home, but now spends 10 hours a day rummaging through the blue rubbish bins that the city has distributed to collect recyclable trash.

Zeeshan said he is able to make a couple of dozen euros per day.

A 22-year-old collector from Ivory Coast, Charles, said: "It is really the only way to make a little money, but it is very hard."

He has been in Greece for just a month, but compatriot Mamadou, 32, backed up that statement on the basis of his six months in the country.

With a room shared with four or five others for 250 euros per month, plus utilities, Charles, who said he used to work as a florist at the presidential palace in Ivory Coast, cannot send any money to his two children.

At least the collectors are not harassed by police.

"When the police see you have a caddy and are trying to work, they hassle you less," Charles said.

The issue of illegal immigration has been the source of heated discussion ahead of Greek legislative elections on May 6, along with the country's debt crisis and related social topics.

Athens official Andreas Varelas said the informal recycling network run by immigrants "disorganizes the traditional recycling circuit," which already lags behind the EU norm.

"The materials are recycled without any quality control," he said, while acknowledging that the activity was the only means of survival for some immigrants.

They are well advised to keep an eye on their trolleys meanwhile. The city has collected 2,000 of them since the start of the year.