NEW YORK, May 30 (Reuters) - Donald Trump became the first U.S. president to be convicted of a crime on Thursday when a New York jury found him guilty of falsifying documents to cover up a payment to silence a porn star ahead of the 2016 election.

After two days of deliberation, the 12-member jury pronounced Trump guilty on all 34 felony counts he faced.

Trump watched the jurors dispassionately as they were polled to confirm the unanimous verdict.

Justice Juan Merchan set sentencing for July 11, just days before the Republican Party is scheduled to formally nominate Trump for president ahead of the Nov. 5 election.

The crime of falsifying business documents carries a maximum sentence of four years in prison, though those convicted often receive shorter sentences, fines or probation. Incarceration would not legally prevent him from campaigning, or taking office if he were to win.

He will not be jailed ahead of sentencing.

The verdict plunges the United States into unexplored territory ahead of the November vote, when Trump will try to win back the White House from Democratic President Joe Biden.

Trump, 77, has denied wrongdoing and an attorney representing him said they would appeal as quickly as possible.

"This was a disgrace," Trump told reporters afterwards as he proclaimed his innocence and repeated his complaints that the trial had been rigged against him.

"The real verdict is going to be Nov. 5 by the people," he said.

Trump gave a thumbs-up sign through the tinted window of his SUV as his motorcade left the courthouse. Trump supporters stood in a park opposite the courthouse along with journalists, police and onlookers.

Opinion polls show Trump and Biden, 81, locked in a tight race, and Reuters/Ipsos polling has found that a guilty verdict could cost Trump some support among independent and Republican voters.

The case had been widely regarded as the least consequential of the four criminal prosecutions Trump faces. But the verdict looms large now as it is likely to be the only one before the election with the others delayed by procedural challenges.

The jury found Trump guilty of falsifying business documents after sitting through a five-week courtroom presentation that featured explicit testimony from porn star Stormy Daniels about a sexual encounter she says she had with Trump in 2006 while he was married to his current wife Melania. Trump denies ever having sex with Daniels.

Trump's former fixer Michael Cohen testified that Trump approved a $130,000 hush money payment to Daniels in the final weeks of the 2016 election, when Trump faced multiple accusations of sexual misbehavior.

Cohen testified that he handled the payment, and that Trump approved a plan to reimburse him through monthly payments disguised as legal work.

Trump's lawyers hammered Cohen's credibility, highlighting his criminal record and imprisonment and his history of lying. Merchan also cautioned jurors to examine his testimony carefully.

The relatively short amount of time jurors needed to reach a verdict was a sign that they thought there was enough evidence to back up Cohen's testimony, said George Grasso, a retired New York judge who attended the trial.

A source familiar with the Trump campaign's inner workings said the verdict was expected to prompt him to intensify deliberations on picking a woman as his vice presidential running mate. His campaign website labeled him a "political prisoner" and urged supporters to donate.