Scotland Yard said a 55-year-old man is being questioned at a south London police station after an incident in Abbeywood, south east London, today.
Eight people were taken to hospital suffering breathing difficulties after they were found inside a German chemical tanker.
Emergency services were called to Harrow Manorway, Bexley, after the driver apparently stopped to check his load shortly after 8am.
Police said the group were suffering breathing difficulties as a result of gas created by the load, a powdered carbon-based chemical.
Witnesses described a strong smell, similar to that created by barbecue brickets, but police have not confirmed what the tanker was carrying.
Fire brigade officials inspected the tanker, which had stopped in an underpass beneath Eastern Way, before it was driven away by police.
The tanker is owned by H Freund, a German company with offices in Frechen, west Germany, and Runcorn, Cheshire.
The company said it was liaising with the Metropolitan Police over the circumstances of the incident. An employee declined to comment further.
Lorraine Johnson, 57, watched from outside her house as the stowaways were removed by emergency workers.
She said: "They climbed on top of the vehicle and the ambulance turned up and they were just pulling out these people.
"There were eight or nine of them, all laid out over there. Then the helicopter landed and they were just taking them one by one.
"The police kept saying 'Go inside, go inside'.
"I said 'What's going on?', and he (a policeman) said 'It is illegal immigrants from eastern Europe'."
Mrs Johnson continued: "I thought they were all dead, they were just limp... they were just pulling them up, four of them, one on each leg, one on each arm, pulling them out."
The occupants, whom she described as having dark skin, were then given oxygen and taken towards the ambulances.
Local workmen, constructing a new water main, reported hearing banging coming from the back of the tanker.
Pete Hobbs and his son, also called Pete, were among those who saw the apparent stowaways being taken out of the tanker.
Mr Hobbs jnr, 19, watched as the occupants were taken through a hatch in the top of the tank before being treated at the roadside.
His 40-year-old father, from Chatham, Kent, said: "All you could hear was banging on the side of the tank. The other lads stopped the lorry driver and said 'we can hear banging'.
"He had earplugs in his ears, he got out, I don't know what happened (to him) after that."
His son added: "They were all covered in black stuff, whatever was in the lorry was all over them.
"They weren't bleeding or anything, it looked like they were burnt but I think it was like soot."
A spokeswoman for the Home Office's Border and Immigration Agency said: "We are investigating an incident with Metropolitan Police involving eight suspected illegal immigrants in south-east London.
"The BIA has already introduced a number of measures to make our borders more secure.
"There will inevitably be some occasions when illegal entrants manage to penetrate our borders unlawfully, and some do so in lorries.
"But our deployment of state-of-the-art detection technology, the levying of civil penalties on hauliers, and closer co-operation with carriers, port authorities and EU counterparts are clamping down on clandestine entry into the UK."
She added: "Last year overseas immigration controls in France and Belgium stopped 17,000 people crossing the channel illegally and refused over 6,500 people entry.
"Over the next 12 months, the Government plans to introduce measures including creating a single border force bringing together the Border and Immigration Agency, Customs and UK Visas providing a tougher, highly visible policing presence at Britain's ports and airports."
Hauliers can be fined if they have innocently carried illegal immigrants across the UK border after failing to introduce effective systems to keep their vehicles secure.
If a vehicle is found to have clandestine passengers, the driver, employer, owner or hirer may be liable for a penalty of up to £2,000 for each person carried.
If there is evidence a driver - or another party - knew about the illegal immigrants they could face prosecution for people trafficking.
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