Bricks and smoke bombs were thrown during clashes between far-right protesters and anti-fascist groups at an anti-immigration rally in Dover.
Flag-waving demonstrators from the National Front and the right-wing South East Alliance marched through the seaside town to protest against the arrival of immigrants.
They clashed with anti-racism activists including the Kent Anti-Racism Network, who organised a counter-demonstration and were addressed before the march by Diane Abbott, shadow secretary of state for international development.
Police in riot gear and dog handlers stemmed the violence, but at one point officers were pushed back by left-wing activists in black hooded tops and with covered faces who broke through their lines.
There was at least one injury, with a man left with a blood-covered face.
Kent Police described the protest as a "fast-moving and ongoing incident" but were unable to say whether there had been any arrests.
Businesses and residents were warned in advance of disruption, with roads closed and some rail replacement bus services suspended.
Police also said they would be using "stop and search" powers in the area between 9am and 7pm on Saturday.
Before the demonstration the South East Alliance, which describes itself as an "angry, white and proud" street movement, advertised the protest on its Facebook page, saying: "Remember we are there for a purpose. To highlight certain issues we face. We are not there to have a kick-off with the red scum but we do know they will attack us and we shall defend ourselves without hesitation."
And following the march the National Front posted on its own Facebook site, saying: "A big well done and thank you to all white nationalists who attended Dover today to save our country from invasion. Respect to all in attendance."
Earlier, Maidstone services on the M20 were closed after several coaches believed to be taking people to the protest were damaged.
Police were called to a disturbance at the services at junction eight shortly before 11am, where two coaches had swastikas painted on them in blood.
One had its windscreen smashed as rival groups clashed.
Kent Police said one man was arrested on suspicion of possessing an offensive weapon, and another coach that left the scene was stopped by officers at junction 11 services at Westenhanger.