Homeland Security netted more than 680 immigrants in a new series of raids, Secretary John F. Kelly said Monday, saying they were standard enforcement actions that generally nabbed serious criminals. Three quarters of those arrested in the raids — the first major announced action under President Trump — had criminal convictions, Mr. Kelly said. He was responding to claims of immigrant-rights advocates, who said the administration was spreading fear by targeting rank-and-file illegal immigrants who have generally stayed out of serious trouble with the law, save for living and working here illegally.