Violence broke out between rival demonstrators travelling to and attending pro- and anti-refugee rallies in Dover on Saturday.
Bricks were allegedly thrown as anti-fascists and far-right supporters, including the National Front, came together during the protest and counter-protest.
Kent Police said three people had been arrested during the protests: a 41-year-old man from Gillingham on suspicion of possessing an offensive weapon; a 32-year-old man from Bristol on suspicion of breaching the peace; and a 28-year-old man of no fixed address on suspicion of a public order offence.
One person suffered a broken arm while five others sustained minor injuries.
Shadow international development secretary Diane Abbott had addressed the anti-racism demo.
Earlier, anti-fascist activists and far-right supporters clashed en route to the demonstrations, with a motorway services station in Maidstone closed when coaches carrying rival activists arrived at the same time.
Anti-fascist demonstrator Anindya Bhattacharyya said that a "coachload of Nazis" arrived at the services and attacked the coach he was travelling on.
He shared a picture of a swastika that had been daubed on his coach, which another activist said had been drawn in blood.
Kent Police told BuzzFeed News officers were called to a disturbance at 10:51am at the junction 8 services on the M20.
"A number of coaches were damaged and six people suffered non-life threatening injuries," a spokesperson said.
Six men aged between 25 and 53 were arrested on suspicion of violent disorder.
Police said 20 weapons were seized in Dover and the M20 services, including a lock-knife, knuckle duster, poles "adapted to cause harm", pieces of wood, glass, hammers and bricks.
Video footage from Dover had showed violence breaking out and smoke bombs being let off.
Matthew Champion is a deputy world news editor for BuzzFeed News and is based in London.
Contact Matthew Champion at firstname.lastname@example.org.
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