The 13 people, believed to be from Ethiopia, were seen jumping out with luggage near Chertsey at around 9am, before the HGV was driven away.
Police were called to the scene and the group were found walking alongside the busy motorway.
Motorists saw them being arrested and searched, and the incident caused traffic tailbacks in both directions of the M25 between junctions 12 and 13.
The lorry involved was believed to have come from mainland Europe.
A Surrey Police spokesman said: “We received a number of calls around 8.50am from members of the public reporting that they had seen a large group of people getting out of the back of a lorry which had stopped on the hard shoulder between junctions 11 (Chertsey) and 12 (the M3).
“The lorry was driven off prior to officers attending the scene. However, the 13 people, including men and women, were located walking along the M25 and were arrested on suspicion of immigration offences.
“They are currently in custody and the UK Border Agency has been informed. It is believed the group is from Ethiopia.”