Barack Obama gave an emotional thank you speech to a group of campaign volunteers in Chicago, a day after his re-election as U.S. president and defeat of Republican presidential nominee Mitt Romney in a tight race.

"What you guys have done means that the work that I'm doing is important," Obama said, choking back tears. "And I'm really proud of that. I'm really proud of all of you."

He then wiped away a tear, prompting cheers and applause from the crowd.

"What you guys have accomplished will go on in the annals of history, and people will read about it and they'll marvel about it," he said in the speech posted to YouTube.

"But the most important thing you need to know is that your journey is just beginning. You're just starting, and whatever good we do over the next four years will pale in comparison to what you guys end up accomplishing for years and years to come."