U.S. President Barack Obama surprised guests at a fundraising event at the Apollo Theatre on Thursday evening by singing a line from the Al Green classic Let's Stay Together.

The song's title might fit well with a president seeking re-election. But the opening line Obama chose to sing was a little more personal.

"I ... I'm so in love with you," Obama sang to cheers and applause, after taking the stage and calling out, "To know that Rev. Al Green was here!"

Green was one of the celebrities in attendance at the event in Harlem, N.Y.

It was one of several fundraisers attended by Obama on Thursday, according to media reports. He also reportedly appeared at a party hosted at the home of director Spike Lee that cost attendees more than $35,000 US per ticket.