The remarks were a defiant response from Mr. Trump, who had remained uncharacteristically silent on social media during Mr. Comey’s blockbuster day of testimony on Thursday, as the former F.B.I. chief laid out an account that strongly suggested the president’s private exchanges with him had been an attempt to obstruct justice. They escalated an extraordinary public feud between a sitting president and the ousted F.B.I. director who had been investigating his campaign, each now engaging in full-throated accusations that the other is lying.