Authorities abruptly shut down the Dominican Day Parade in Manhattan on Sunday when a massive, unruly crowd swarmed the Midtown route and a teen slashed a 65-year-old man across the face, law-enforcement sources said. The 42-year-old parade — which draws a half-million spectators and features 113 organizations, including community groups, carnival bands and government officials — was supposed to continue along Sixth Avenue until West 59th Street, the entrance to Central Park.