A lorry driver is being questioned after police stopped a vehicle on the M6 near Stoke and found 27 suspected migrants in the back.
Staffordshire Police arrested the man, 42, on suspicion of “facilitating the illegal entry” of people into the UK.
Officers acted after receiving reports flagging up concerns about the welfare of those in the wagon, the force said.
It is understood they were medically assessed and transferred to immigration officials for interview.
Part of the motorway was closed from about 16:00 GMT – causing lengthy tailbacks – as police searched vehicles near Keele services.
The lorry was stopped on the northbound carriageway between Junctions 15 and 16 at around 17:45.
Driver Danny Ellis said he saw 30 to 40 police cars pass him on the northbound side of the motorway, and said three or four lorries were stopped and searched.
“We were told for safety to get back in our cars and lock them,” he said.
“We were told a lot of people escaped and ran from the back of these lorries.”
The 30-year-old telecoms worker said he could see men on the ground being treated by paramedics.
Another eyewitness told BBC Radio Stoke he saw police stopping lorries.
He said: “They pulled a white trailer up on the side and they were shouting ‘are they in here?’
“The driver opened the vehicle and I just watched 15 of them come out.”
Police said a number of people fled but were eventually detained, with none reporting serious injuries.
Deputy Chief Constable Nick Baker, said: “We were acting on information where there was real concern for people in distress and therefore it was necessary for us to take immediate and appropriate action.”
He apologised to motorists for disruption on the motorway, which Highways England said was not fully reopened until about 22:00 GMT.
Among those caught up in huge tailbacks were hundreds of Burton Albion supporters who had been travelling to their team’s Carabao Cup semi-final against Manchester City, which they lost 9-0.
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: