Nearly a hundred French fishermen rallied at Boulogne-sur-Mer, Europe's largest seafood processing center, in northern France on Thursday.
They say they've been denied the right to fish in UK waters, and started fires and blocked trucks carrying fish from the UK in protest
One sign read - "You want to keep your waters??? OK ... So, keep your fish!!!"
Britain's post-Brexit trade deal with the European Union only allows the bloc's fishermen to access British waters with a license.
French fisherman Bruno Margolle says those licenses were expected to be issued within days, only to drag on for months.
"On the evening of December 24, everyone was relieved that we had finally got a deal. On January 1, we had the assurance that within 48, 72 hours, everyone would get their licenses to operate within the UK's 6-12 mile zone. As of today, only 22 out of 120 boats have received their licenses."
Margolle says many of those still struggling to obtain a license are unable to meet a British demand in the trade deal.
That condition seeks proof that the skippers have fished in UK waters during the five years running up to Britain's 2016 referendum on EU membership.
Britain claims it maintains an evidence-based approach to licensing EU vessels using information supplied by the European Commission.
A British government spokesman called Thursday's protest "unjustified," and said it's raised those concerns with French authorities.
Meanwhile the French government said late on Thursday that the European Commission must ensure Britain holds up its side of the deal, citing the "urgency of the situation."
About two-thirds of fish from the UK are exported to the EU.
French fishermen say the country's fish stocks might be depleted if they still cannot cross into British waters.