U.S. officials briefly shut down one of the world's busiest border crossings Monday to set up concrete blocks and barbed wire as Central American migrants arrive.The closure of the crossing between Tijuana, Mexico, and San Diego, California, kept a large number of Mexicans from entering the U.S. where there are jobs.It also caused a massive traffic backup for U.S. citizens trying to get back home after visiting Mexico.The barriers are an apparent precaution against fears that a large number of people may try to rush the border to get into the U.S. About 3,000 migrants from the various Central American caravans are already in Tijuana and thousands more are expected throughout the week.They have gotten a hostile reception from hundreds of locals in Tijuana, who greeted the migrants with cries of "get out" and "go home."Some echoed President Donald Trump's rhetoric, calling the Central Americans criminals and the caravan an "invasion."Trump tweeted Sunday that the U.S. is "ill-prepared for this invasion and will not stand for it. They are causing crime and big problems in Mexico. Go home."But some of the migrants are puzzled by fears they are dangerous, saying they fled Honduras for the U.S. to escape violence and gangs.