Masked demonstrators are preventing traffic from accessing the Port of Dover.
Kent Police said it was aware of a protest “currently taking place” at the ferry terminal and was “engaging with protesters”.
Traffic has been stopped from going in and out of the port.
Images from the scene appear to show hundreds of protesters, all dressed in black and wearing face coverings.
P&O Ferries said it would get passengers on to the next available sailing if they missed their scheduled crossing.
Ferry operator DFDS said it was aware of the “current traffic disruption” and would accommodate passengers on the next departure once they arrived.
Traffic is stopped in both directions on the A20, between the A256 York Street and the A2 Jubilee Way in Dover, according to National Highways.
It is reporting four miles (6.4km) of congestion and delays of up to 30 minutes.
