French authorities arrested more than 450 people on Thursday in the most violent day of demonstrations since the start of the year against the bill to raise the retirement age from 62 to 64. Rights groups have raised concerns over the repeated use by police of "kettling", also called "trap and detain" in the United States, a crowd-control tactic consisting of cordoning off protesters in a small area. Right groups have long accused French police of brutality and racism, and say internal investigations seem to result in few sanctions