Huge queues lined major routes near the coast as Operation Stack came into force.
Motorists heading to the Port of Dover faced miles-long queues as police check all vehicles and passengers bound for the continent.
The enhanced security measures were requested by counter-terror police on Tuesday.
Operation Stack - an emergency measure designed to prevent gridlock in Kent - has been activated on the M20 to allow lorries to line up to enter the port.
But pictures show long lines of HGVs bound for Dover and the Channel Tunnel.
All other traffic is being diverted off the M20 at junction eight, with lorries the told to join Operation Stack at the same junction.
During Operation Stack, freight is separated into two queues on either side of the coast-bound carriageway.
One lane is used for traffic heading to the Eurotunnel terminal and one lane is for port traffic. The middle lanes are kept clear for emergency vehicles.
Traffic for east Kent is being advised to use the A2 and M2, Kent County Council said.
Counter Terrorism Policing said it had requested enhanced security checks at all UK ports in relation to "ongoing operational activity".
"The requirement has since been stood down," it added.
Kent Police said border checks were returning to normal.
Increased checks were also being carried out at the Eurotunnel terminal in Folkestone, a spokeswoman for the Port of Dover said.