The civil disobedience demonstration is the latest by Insulate Britain, a new group whose activists have repeatedly blocked traffic on London's busy M25 orbital motorway.

Police in Kent, southeast England, said the 39 people were detained on suspicion of causing a public nuisance and obstructing a highway, after officers were alerted to people blocking roads near the port on Friday morning.

Insulate Britain said that more than 40 activists had obstructed the approaches to the eastern and western docks at Dover.

The port, which sits on the Channel coast less than 30 miles (50 kilometres) from France, handles around 17 percent of Britain's goods trade.

Traffic was brought to a standstill, but officials said the port itself remained open.

Kent police chief superintendent Simon Thompson said his officers were working with other forces, prosecutors and partner agencies "to gather evidence and ensure there are consequences for those who break the law".

Insulate Britain earlier apologised for the disruption but said it was "the only way to keep the issue of insulation on the agenda".

"We are blocking Dover this morning to highlight that fuel poverty is killing people in Dover and across the UK," a spokesperson said.

"We must tell the truth about the urgent horror of the Climate Emergency. Change at the necessary speed and scale requires economic disruption. We wish it wasn't true, but it is."

Mirroring the disruptive tactics adopted by the global Extinction Rebellion grouping in recent years, Insulate Britain has blocked motorway traffic five times since mid-September.

On Wednesday, the government won a court injunction over the M25 protests, meaning activists could be imprisoned if they block Britain's busiest motorway.

Political leaders including interior minister Priti Patel and Transport Secretary Grant Shapps have decried their tactics, warning they risk lives.

The protests come as Britain prepares to host the UN climate change conference COP26 in Glasgow in November, with hopes of firmer commitments to prevent runaway global warming.