Exports of fresh fish and seafood have been severely disrupted by new border controls since the UK's transition period ended earlier this month.
A BBC reporter said between 15-20 lorries had been driving up Whitehall in central London.
The prime minister has promised compensation for affected firms.
Industry associations have complained that extra paperwork has made it difficult to deliver fresh produce to mainland Europe before it goes off.
On Sunday, Foreign Secretary Dominic Raab said ministers were working with the sector to resolve "teething problems" faced by exporters."
Brexit: Fishing firms hold London protest over disruption - BBC News
