Dozens of people dressed in black and wearing balaclavas have blocked the road at the port of Dover in Kent, forcing closures in both directions.

Kent police said they were aware of the protest, which the Port of Dover described as a “public order incident”. Officers were in attendance and engaging with protesters, they said.

The behaviour of the participants and the travel disruption they have caused has been condemned by the government. Traffic in and out of the port on the A20 has been stopped, causing delays of up to 30 minutes and four miles of congestion, according to National Highways.

Videos and pictures shared online show a large number of masked people in black marching on the road in a large group as lorries wait in traffic next to police cars.

Access to the port was partially restored via the M2/A2 by 10.30am, but access via the M20/A20 remained blocked, according to P+O Ferries.

The number of people involved is not year clear. One of those livestreaming the event estimated he was marching alongside hundreds of people. The Press Association reported that there were dozens in attendance but police have not given an official estimate.

Videos circulating on social media show demonstrators linking arms in a blockade across the A20 and shouting “stop the boats”. They could be seen marching along the A road, chanting: “England till I die.”

A group called Patriot Platform, which styles itself as having “a fierce love for our homeland”, appeared to claim responsibility for the blockade on social media.

The far-right activist Tommy Robinson, whose real name is Stephen Yaxley-Lennon, reposted multiple videos of the blockade, highlighting a livestream of the protest. The livestream directed viewers towards Patriot Platform.

In his post, Robinson encouraged more to join, saying “get there to support them”.

In another post, he wrote: “Simultaneously at different locations across Dover patriots have mobilised to close down the port. There are 3 plus locations that patriots have shut down so far with many more planned to be closed. People wanted action I’m guessing there are 100,000’s of men willing to fill their shoes.”

The founder of Patriot Platform, Danny Thomas, also known as Danny Tommo, wrote on social media “live Dover somethings [sic] happening” and posted a livestream.

In a video posted on 2 September, he told his followers he was meeting with “a huge group [of] top, proper patriots” to prepare for a message he was planning to send to the government.

“Very, very soon, we are going to send a message that will make sure they understand that we will not go down without a fight. We will not allow our nation to be plagued in the way that it is. And when you see it and when you feel it, I guarantee the whole thing will change for the better. Because once the message is sent once, imagine the sheer numbers that will be sent on the second, the third, the fourth message.”

In further comments, he said: “From the hills of Scotland to the fields of Wales to the streets of Northern Ireland and to the towns and cities of England, we are building something that’s never been built before. And when you’re part of it and you see what’s about to go down, you will understand … we’re about to show you what can be done when men and women of this country, who have the blood that runs through them like their grandfathers, get off their asses and do something about it.”

On Saturday morning, the Port of Dover confirmed in a statement that the disruption was affecting “all routes” in and out, as well as around the town, and that it was in touch with the police.

It said: “The Port of Dover can confirm that there is an ongoing public order incident happening around the town of Dover. This situation is impacting our local community and our customers, so of course we are hoping for a swift resolution.”

Nathan Ditchburn, who was on his way to work when he saw the group arrive at about 6.45am, told KentOnline: “I would say there are around 300 men, maybe more. Nobody will speak; they just handed me a piece of paper to look at. They have blocked off every entrance and exit of the Dover port area, and Jubilee Way is blocked as well. It will be causing huge delays for the ferries today.”

The local parkrun group cancelled its Dover Waterfront run, telling runners the course was unsafe or blocked. A spokesperson told KentOnline: “I am afraid we are going to cancel this morning. We have mass disruption with some protesters.”

Mike Tapp, the Labour MP for Dover and Deal, criticised what he described as “idiots coming to Dover to cause disruption”. In a post on Facebook, he said: “Parkrun cancelled and port access disrupted. I am fed up with idiots coming to Dover to cause disruption. I bet if you asked any one of those ‘protesters’ they wouldn’t know small boat crossings are down by 40% and we are working hard to fix the mess. Go home, enjoy your weekend/get a job.”

Chris Vinson, leader of the Conservative group on Dover district council, said the protest appeared to be “illegal migration-related”, but said the demonstrators did not appear to have placards or banners.

“I’m not aware that there’s been any violence, although clearly, having the path of your vehicle blocked by men wearing black paramilitary gear and balaclavas is extremely intimidating, especially when it’s not clear why they’re there, and the police weren’t aware and so haven’t been able to warn people in advance and make sure that they are made to feel safe,” he said.

“I know people who have worked night shifts in the port who are being prevented from leaving for fears for their safety.”

A local Reform UK councillor, Paul King, told residents to avoid the port area. : “I’ve been told it’s a stop the boats protest.” he said.

A Home Office spokesperson said: “The right to peaceful protest is fundamental to our democracy, but we condemn the behaviour in Dover and the travel disruption it has caused. We appreciate the frustration of those who have been impacted today. Access to the port is now restored and traffic is flowing. Those travelling to the Port are advised to allow for extra travel time and to follow guidance from providers.

“We thank policing partners and travel providers who we continue to work with to ensure the safety of passengers and the wider community.”

The shipping company DFDS said: “We are aware of the current traffic disruption affecting access to the port. Rest assured we will accommodate you on to the next departure once you arrive.”

P&O Ferries said its sailings were still going ahead as scheduled, adding that those who missed their departure would be offered the next available crossing.