Hundreds of Central Americans arrived at the United States's southern border Wednesday, defying visiting US Defense Secretary Jim Mattis, as the rest of a migrant caravan dramatically accelerated its pace to join them. Large groups of migrants gathered on the beach in the northern city of Tijuana at the border fence that separates Mexico from California, climbing on top of it and even jumping down to the other side as US border patrol agents looked on -- before quickly scrambling back to the Mexican side. "We're not criminals!" some of them shouted, in a pointed message to US President Donald Trump, who has deployed some 5,900 troops to the border as the caravan approaches.