LONDON — The British capital was tense Wednesday as police, fearing a possible outbreak of the right-wing rioting that has erupted elsewhere in Britain in recent days, flooded into several neighborhoods with large immigrant populations.

Joining them in neighborhoods like Finchley and Harrow were hundreds of counterprotestors carrying signs that said, “Refugees Welcome” and “Racists Out, Refugees In.”

“I’m here to stand up for human rights,” said a 25-year-old plumber, Jones Percival….