Aged 35 and 53, the men had broadcast content likely to incite hatred live on a YouTube channel, according to police sources.
Two British nationals suspected of coming to France to take part in a banned far-right protest against migrants were arrested and taken into custody, French authorities said on Monday.
The two men, who were broadcasting live videos from the French coast, were arrested near the northern town of Calais on Sunday evening, Pas-de-Calais prefect Francois-Xavier Lauch told the AFP news agency.
Immigration has become a central political issue in France and the United Kingdom, where the government seeks to stop undocumented migrants arriving on British shores after paying smugglers to cross the Channel.
The two men were detained for inciting hatred and participating in a group with the aim of preparing acts of violence based on comments made on social media, said Boulogne-sur-Mer prosecutor Cecile Gressier.
These are the first arrests of British far-right activists in France on such grounds, she said, adding that they are not accused of physical violence.
They were not among the 10 far-right activists banned from French territory since mid-January by the interior ministry on charges of "violent actions" against migrants in northern France.
The two may be sent to an administrative detention centre before being deported where they will be held with other migrants ordered to leave France.
Aged 35 and 53, the men had broadcast content likely to incite hatred live on a YouTube channel, according to police sources.
The arrival of Syrian, Iraqi and Sudanese migrants has fuelled public concerns and rising anger on the British far right and since last year videos have circulated of anti-migrant vigilantes visiting France to take matters into their own hands.
French officials have announced a ban on an anti-migrant rally called "Operation Overlord" launched by British far-right activist Daniel Thomas.
The name refers to World War II-era Operation Overlord when tens of thousands of Allied troops stormed the beaches of Normandy in northern France on 6 June 1944, paving the way for liberation and the end of the war against Nazi Germany.
Call for protests
In several videos broadcast live on a YouTube channel on Sunday, two British men, presumably those who were later arrested, can be seen walking along a beach near Calais.
"I'll guard the beaches tonight, if no one else wants to," one of them said. He called on other British people to join him in limiting illegal arrivals, without violence.
"It's not until you get here that you realise the scale of what is going on and how protected they are as well," the man added. "There is only a certain amount that you can take, enough is enough."
Thomas had called on social media for rallies at the weekend in the port of Dover in southeastern England and on the northern coast of France, which he said were necessary, claiming French authorities are unable to prevent illegal crossings.
Thomas posted images of himself waving a flag on Saturday and Sunday, which he says were taken over the weekend in France.
Around a hundred people also gathered on Saturday in Dover, where most migrants land once they have been intercepted by the British authorities.
Thomas, who claims to be subject to a French travel ban himself, has not been arrested, according to the prefecture.
According to the French authorities, Thomas leads a "very radical" branch of the "Raise the Colours" movement, which was formed following a split after the French entry ban.
Anti-racism campaigners say far-right activists are behind the "Raise the Colours" movement.
The main "Raise the Colours" account said in a post on X on Saturday it had nothing to do with the operation led by Thomas.
In October, the French judiciary opened a preliminary investigation into "aggravated violence" committed against migrants and reported by a rights association, which suspects British far-right activists, though it does not have "formal proof."
Last year saw the second-highest number of undocumented migrants arrive on British shores since such crossings began in 2018.