Tensions escalated sharply on Thursday as Donald Trump’s lawyer fiercely countered prosecutors, repeatedly accusing their key witness of lying.

In the most intense cross-examination yet, Michael Cohen, Mr. Trump’s former fixer, detailed conversations with the former president regarding a hush-money payment to an adult-film star.

However, attorney Todd Blanche vehemently dismissed Cohen’s testimony, almost shouting that it was “a lie.”

Blanche argued that records indicated Cohen had actually contacted Mr. Trump’s bodyguard about a prank caller, not about the alleged payment.

On Thursday, Mr. Blanche’s theory about the phone call was aimed at casting doubt on Michael Cohen’s credibility during his third day on the stand, as the jury watched the heated exchange with rapt attention.

After the tense confrontation, Mr. Blanche returned to the defense table, sitting beside his client. When the judge called for an afternoon recess, a collective sigh of relief filled the room.

Donald Trump faces 34 felony counts of falsifying business records, accused of disguising payments to Cohen as legal expenses. These payments were allegedly reimbursements for Cohen’s hush-money payment to adult-film star Stormy Daniels, who claims she had an affair with Mr. Trump.

Prosecutors contend that Mr. Trump tried to keep damaging information hidden to protect his 2016 presidential campaign. Mr. Trump has pleaded not guilty to all counts and denies having an affair with Ms. Daniels.

Under pressure on the stand Thursday, Cohen insisted that his previous testimony was accurate and that he discussed the payout to Ms. Daniels with Mr. Trump in a call on October 24, 2016.

Earlier this week, prosecutors questioned Cohen about the call to establish Mr. Trump’s alleged direct involvement in the payoff scheme. Cohen testified that he kept Mr. Trump informed at every step of the process in paying Ms. Daniels.

As the central figure in the payout, Cohen’s testimony is pivotal for prosecutors to demonstrate whether Mr. Trump had knowledge of the purportedly fraudulent reimbursement plan.

However, Cohen’s criminal record, history of lying to Congress, and outspoken public criticism of Mr. Trump make him a contentious witness. In 2018, he pleaded guilty to federal campaign finance violations related to the hush-money payment but insists he acted under Mr. Trump’s direction.

The defense focused on Cohen’s credibility issues for nearly two days, portraying him as a liar with a grudge against Mr. Trump.

On Thursday morning, Mr. Blanche played recordings from Cohen’s podcast, *Mea Culpa*, where Cohen expressed a desire to see the former president go through the booking process, saying, “I want this man to go down.”

Mr. Blanche also confronted Cohen with a social media post where he referred to the former president as “Dumbass Donald.”

“Does the outcome of this trial affect you personally?” Mr. Blanche asked.

“Yes,” Cohen replied.

The president’s attorney repeatedly pressed Cohen about his guilty plea for lying to Congress and aggressively questioned him about previous statements in which he claimed that the tax evasion charges he pleaded guilty to were unfair.

Several hours of similar questioning culminated in a dramatic showdown over the October 24, 2016 phone call.

Mr. Blanche first asked Cohen if he recalled receiving harassing calls in late October 2016. Cohen confirmed that he had.

Mr. Blanche then displayed communications between Cohen and Mr. Trump’s bodyguard, Keith Schiller, on October 24, discussing how to handle the prank calls.

Amid their discussions about the prank caller was a call between Cohen and Mr. Schiller that Cohen previously testified he made to discuss the payout to Ms. Daniels with Mr. Trump. That call lasted one minute and 36 seconds.

Mr. Blanche expressed skepticism that Cohen could have discussed both the prank caller and the six-figure payout in such a brief period.

Cohen countered that part of the call was about “the 14-year-old” behind the prank calls. He knew Mr. Schiller was with their boss at the time, and the call was about more than just the harassment, he said.

Cohen insisted he always ran “everything by the boss” immediately and that he did so on that call.

The response drew a dramatic reaction from Mr. Blanche: “That. Was. A. Lie,” he declared loudly.

Numerous Republican members of Congress filled the benches behind Mr. Trump on Tuesday in a show of partisan support.

A Trump campaign spokeswoman and his son, Eric Trump, were in attendance.

Reps. Matt Gaetz of Florida, Lauren Boebert of Colorado, and Andy Biggs of Arizona were also part of the entourage, which was so large that some lawmakers had to take seats at the back of the courtroom.

Cohen’s testimony has sparked significant public interest. On Thursday morning, the line to get into court stretched down the block, with journalists and members of the public hiring line sitters to secure a spot overnight. One line sitter, whose employer did not show up, offered others in line $400 for her spot.

Despite the overwhelming public interest, the defendant appeared largely indifferent in the courtroom: Mr. Trump sat silently for most of the morning session. Occasionally, he focused intently on the cross-examination.

Although one of his biggest public adversaries sat just a few feet to his right, Mr. Trump mostly stared straight ahead.

“I think it was a very interesting day,” Mr. Trump told the waiting cameras as he left the courtroom. “A fascinating day.”