WASHINGTON — President Trump declared angrily on Thursday that he was the victim of a witch hunt, and all but contradicted his deputy attorney general on the firing of the F.B.I. director, in an extensive denial of any collusion between his campaign and Russia.

Mr. Trump used a chaotic White House news conference with Colombia’s leader to directly confront a weeklong barrage of criticism and questions in a political storm that he said was dividing the nation. He also conspicuously distanced himself from aides like his former national security adviser, Michael T. Flynn, in repeating his claim that Moscow and its agents had not secretly assisted his campaign.

“I respect the move,” Mr. Trump said of the Justice Department’s decision on Wednesday to appoint a special counsel to investigate the matter. “But the entire thing has been a witch hunt.”

“And there is no collusion between, certainly, myself and my campaign — but I can always speak for myself — and the Russians,” he said. “Zero.”