Authorities believe the lorry originated from Bulgaria, entering the UK over the weekend.
"We are in the process of identifying the victims, however I anticipate that this could be a lengthy process," Chief Superintendent Andrew Mariner said in a statement.
They have arrested a 25-year-old Northern Irish truck driver "on suspicion of murder," Essex Police said.
"We have arrested the lorry driver in connection with the incident who remains in police custody as our enquiries continue. I appreciate this cordon is going to disrupt the activity of local businesses in the area and we will work to ensure that disruption is kept as short as possible. We are working with Thurrock Council to mitigate against any impact our investigation scene will have locally, " Mariner added.
"This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened," he added.
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