WATCH: Trump tells RNC audience to 'be nice' after man shouts Obama slur
Donald Trump appears at the Republican National Convention (C-SPAN/screen grab)

President Donald Trump admonished a supporter who slurred former President Barack Obama at the Republican National Convention on Monday.

In what he called a "surprise" appearance on the first day of the convention, Trump accused the Democrats of trying to "steal" the election with mail-in voting.

"What it means is they are trying to steal the election from the Republicans, that's what it means," he told supporters. "Just like they did it last time with spying, and we caught them. And that included President Obama."

At that point a man shouted so loudly that it caused the president to stop speaking.

"Let's be nice," Trump said, acknowledging the slur. "This could only happen in North Carolina."

On Twitter, many commenters accused the Trump supporter of shouting in the word "monkey" in reference to former President Barack Obama.

Watch the video and read some of the tweets below.