Boris Johnson has pledged £23m to help businesses affected by Brexit delays amid protests by fishing firms.
Demonstrations took place outside government departments in central London by exporters who are warning their livelihoods are under threat.
Exports of fresh fish and seafood have been severely disrupted by new border controls since the UK's transition period ended earlier this month.
The PM said firms would be compensated for delays that were not their fault.
Industry associations have complained that extra paperwork has made it difficult to deliver fresh produce to mainland Europe before it goes off.
They have warned that if the situation continues, jobs could soon be at risk.
Pressed on what he would do in response, Mr Johnson said the government would step in to support firms which "through no fault of their own have experienced bureaucratic delays, difficulties getting their goods through, where there is a genuine willing buyer on the other side of the channel".
"There's a £23m compensation fund we've set up and we'll make sure they get help," he said.
Details of the scheme are expected later this week.
After a day of protests in central London, which saw 20 lorries drive up Whitehall, the Metropolitan Police said 14 people had been reported for Covid-related offences, but no arrests were made.
Mark Moore, manager of the Dartmouth Crab Company, said his business and others were protesting to "raise awareness" of the impact of new border checks.
He told BBC Radio 5 Live his company had faced delays of up to eight and a half hours when delivering produce into the European Union.
He added that the situation was "especially difficult" for the shellfish sector, where goods were at risk of going off before reaching customers.
"It's not about the increased documentation per se," he said.
"We have taken that on board, and we ourselves - and I know many others - have had no issues with producing the actual paperwork.
"It's the volume required and the timeframe in which to produce it, which doesn't lend itself to live shellfish and fish generally."
At the scene
John McManus, BBC News
There are 24 lorries in total, overwhelmingly from seafood exporters in Scotland. Businesses taking part say the Brexit trade deal has left their industry high and dry.
And although one haulier from Aberdeenshire I spoke to was keen to stress that their coordinated protest was peaceful, it is clear that they all feel that direct action is now necessary to make the government sit up and take notice.
Good natured though their action was, it did for a time cause serious traffic congestion along Whitehall and Parliament Square.
However, low levels of traffic perhaps caused by the Covid lockdown meant the roads around Whitehall didn't grind to a complete halt.
At stake, they believe, is an industry, but also thousands of livelihoods. Exporters say they are backed by fishermen who are struggling to land their catches.
And although the rural Scottish communities which are sustained by fishing might seem like a long way from the streets of SW1, the hauliers certainly made their presence felt this morning.
Having left the EU's customs union and the single market, UK exports are subject to new customs and veterinary checks which have caused problems at the border.
Some Scottish fishermen have been landing their catch in Denmark to avoid the "bureaucratic system" involved in exporting to Europe, according to Scotland's rural economy secretary.
Last week, Boris Johnson told a committee of MPs that fishing firms impacted by disruption would be compensated for "temporary frustrations".
But the BBC was told that the Department for Environment, Food and Rural Affairs (Defra) did not know about the promise of compensation before it was made by Mr Johnson.
'Failure to prepare'
Speaking to reporters, the prime minister said he understood the "frustrations" of the fishing industry, noting its plight had been "exacerbated by the Covid pandemic".
"Unfortunately, the demand in restaurants on the continent for UK fish has not been what it was before the pandemic, just because the restaurants have been closed for so long," he added.
Labour leader Sir Keir Starmer accused ministers of trying to "blame fishing communities" for problems "rather than accepting it's their failure to prepare".
"The government has known there would be a problem with fishing and particularly the sale of fish into the EU for years," he told reporters.
Concerns felt outside Scotland
By Martyn Oates, BBC South West Political Editor
Much media attention has been focussed on Scotland as this export crisis has unfolded.
But exactly the same problem is rearing its head in the UK's other great fishing stronghold - at the other end of the UK in Devon and Cornwall.
A virtual Who's Who of South West fishing leaders wrote to the environment secretary back in November warning that the new post-Brexit export requirements would have a "seriously detrimental effect" on the industry, claiming this "could be the final straw for many businesses".
Here, too, many fish exports have now ground to a halt and others have encountered obstacles and long delays.
And exporters have reacted angrily to the government's repeated insistence that the issues they've been experiencing over the last two weeks are just "teething problems".