Authorities believe the truck was born in Bulgaria, entering the UK at the weekend.
"We are identifying the victims, but I predict this could be a long process," Chief Superintendent Andrew Mariner said in a statement.
"We arrested the trucker in connection with the incident that remains in police custody while our investigations continue. I appreciate this cordon that will interrupt the activity of local businesses in the area and we will work to ensure that the interruption is kept as short as possible. We are working with the Thurrock Council to mitigate any impact our investigation scene will have locally, "Mariner added.
"This is a tragic incident in which a large number of people have lost their lives. Our investigations are underway to establish what happened," he added.
More to follow.