The first sign that something was wrong inside the large tanker parked under a flyover came at 8.15am yesterday when Pete Hobbs and his colleagues heard a faint banging. Then as they approached the lorry, the plaintive cries from inside became audible. They said: "Help us. We're dying."

Within minutes the full paraphernalia of a major emergency had descended on the underpass in a scruffy corner of south-east London where early yesterday Mr Hobbs and his workmates fitting a new water main unwittingly stumbled upon a new example of the desperate and dangerous lengths that illegal immigrants are prepared to go to in order to reach Britain.

Covered from head to foot in the black charcoal dust that they had hidden in for at least 12 hours, eight migrants – including a 12-year-old boy – emerged from the sealed hatches of the German-registered lorry, gasping for air.

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The men, confirmed last night as two Eritreans, three Iraqi Kurds and three Iranians, collapsed on to the pavement on the edge of a housing estate in Abbey Wood, near Woolwich, in front of astonished emergency workers and locals.

Mr Hobbs, 19, from Chatham, Kent, a contract worker, said: "We'd just started working and we noticed this big lorry parked up under a flyover. At first all we could hear was this frantic banging from inside the tanker. We rushed over and we could hear men's voices shouting 'help us' and 'we're dying'. We banged on the cab door and got the driver out. He was wearing ear plugs so he couldn't hear anything. He said to call the police and soon afterwards these guys started climbing out. They were totally black from whatever was inside. It was a weird sight. They were coughing and wheezing and the ambulancemen laid some of them out on the ground to help them. They were wrapped in blankets and looked in a bad way."

An air ambulance, a chemical response team and about a dozen police, fire service and ambulance vehicles were called to the scene amid concern that the tanker was carrying a hazardous substance.

In reality, it was carrying about 15 tonnes of charcoal powder for use in the Crossness sewage treatment works, less than a mile from the underpass.

It is understood the migrants were lying on top of the cargo but had sunk into powder and run short of oxygen after breathing all the air in the sealed tank.

While many thousands have risked their lives – and unknown numbers been killed – by clinging on to lorry axles or huddling under the wind deflectors above drivers' cabs, anti-trafficking campaigners said it was the first time that illegal immigrants have been found inside a fully loaded chemical tanker. The 55-year-old driver of the lorry, owned by a German freight company called H Freund, was last night being held at a police station in south London on suspicion of people trafficking.

Scotland Yard said the lorry had begun its journey at the company's headquarters in Cologne, but it was not known where or how the migrants had got inside. H Freund declined to comment on the incident and said it was co-operating with the authorities.

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It is understood the vehicle made its journey via Calais, where hundreds of asylum-seekers attempt to board lorries every night.

Lorraine Johnson, 57, who lives opposite the flyover, said: "The ambulancemen and police were pulling these people out. I thought they were all dead, they were just limp. It took four of the emergency guys to pull one up out of the hatch."

The men were taken to hospital and treated for breathing problems before being discharged into the custody of the Border and Immigration Agency (BIA).

Migrants pay between £2,000 and £20,000 to trafficking gangs who smuggle them across Europe into the UK. The BIA said last month that about 18,000 potential illegal immigrants had been stopped trying to enter Britain last year, of whom 13,000 had been found hidden on vehicles travelling through ports.

Nick Kinsella, head of the UK Human Trafficking Centre, which highlights the plight of such people, said: "This is about criminals selling people in the same way that they sell any other commodity and there is good money to be made."

A perilous journey

* The worst human trafficking tragedy in Britain was in June 2000 when 58 Chinese illegal immigrants were found suffocated in a lorry at Dover docks.

Perry Wacker, 41, a haulier from Rotterdam, closed the only air vent on the container of his lorry to prevent customs officials from discovering his illicit cargo, each of whom had paid thousands of pounds to a criminal gang to be brought to Europe. Officials found the bodies of 54 men and four women hidden behind a consignment of tomatoes. Wacker was jailed for 14 years.

* In December 2001, eight bodies were found in a sea container in Waterford in the Irish Republic. The victims, thought to be from Turkey, Romania and Albania, had suffocated inside a consignment of office furniture from Italy. It is thought they had spent four days at sea on the journey from the Belgian port of Zeebrugge and died after the voyage was delayed for 48 hours by bad weather. A lorry driver taking the container to nearby Wexford was alerted by the banging of five survivors on the sides.

* A tragedy was narrowly averted in October 2003 when 26 Sri Lankan refugees were found suffering the effects of breathing in engine fumes in a van travelling through Dover. Officials said they would have lapsed into unconsciousness after another hour in the vehicle.