LONDON,— Tens of thousands of demonstrators filled central London on Saturday in one of the largest right-wing rallies Britain has seen in recent years, as police reported violent clashes and dozens of injuries. The Metropolitan Police said about 110,000 people took part in the “Unite the Kingdom” march, an event organized by anti-immigration activist Tommy Robinson. Officers reported being punched, kicked, and hit with bottles and flares. Reform UK, the anti-immigration political party that has led recent opinion polls, has not aligned itself with Robinson, keeping a clear distance from his activities. Across parts of England, red-and-white flags have appeared on houses, lampposts, and painted on roads.