The demonstrators, dressed in black and wearing balaclavas, chanted "stop the boats," the BBC reported, in a reference to asylum seekers arriving on the south coast of England from France.
The National Highways agency said the protest blocked all routes to the port and caused a 6.4km traffic jam.
Major roads to the Kent port were blocked on Saturday morning but access to the port had been restored and those involved in the "spontaneous protest" had been dispersed by midday, Kent Police said in a statement on X.
No arrests were made, police said.
Earlier on Saturday, a group was seen marching through the town and chanting "Whose streets? Our streets!" and "stop the boats" as dozens of police officers followed.
Videos showed dozens of protesters milling around roads and roundabouts near the major port, forcing vehicles to a halt.
Others showed demonstrators linking arms in a blockade across the A20, shouting "stop the boats".
Masked men could be seen at Dover Priory station about midday boarding a train.
Successive governments in the United Kingdom have struggled to curb the flow of asylum seekers, often coming in overcrowded inflatable boats.
However, the number of arrivals this year until September 1 was 16,513, about 43 per cent lower than the same period in 2025, according to government data cited by the BBC.
"The right to peaceful protest is fundamental to our democracy but we condemn the behaviour in Dover and the travel disruption it has caused," a government spokesman said.
"We appreciate the frustration of those who have been impacted today. Access to the port is now restored and traffic is flowing."
Councillor Chris Vinson, leader of the Conservative group and the member for Walmer on Dover District Council, told the Press Association the demonstration appeared to be a "co-ordinated" group of "several hundred masked men".
The councillor said it had "brought the town to a standstill" and left "stacks of people" stuck in their cars or workplaces.
He added: "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."
"I know people who have worked night shifts in the port who are being prevented from leaving for fears for their safety," he said
"Women who are in cars feeling intimidated, people stuck in traffic - who obviously are without toilet facilities - queues way back on the A20 and the other approach roads into Dover."
Dover, about 110km southeast of London, dealt with more than nine million passengers and two million freight vehicles in 2025, according to the port's website.
with PA and AP
