Beyonce has branded Willie Nelson "the coolest

Sharing a series of photos from the event, including some in which she supported one of Willie's iconic red bandanas, Beyonce wrote on Instagram: "The Great Willie! You are the coolest!

"I admire you, and I sincerely thank you for being the pioneer you are. Thanks for your impactful music, your advocacy, and the gracious way you appreciate and celebrate other artists! You are 1 of 1. (sic)"

The 43-year-old star recalled how she and her Destiny's Child bandmates had "fanned out" during their first meeting with the country legend several years ago.

She added: "You were so kind to me and the girls of Destiny's Child when we fanned out meeting you years ago.

"And you greeted me with the same kindness 20 years later!

"You are one of our national treasures.

"Sending my love to you."

During the rally, Beyonce, along with her former bandmate Kelly Rowland, endorsed the current vice president for the upcoming presidential election.

Speaking on stage, the 'Crazy in Love' hitmaker - who has Blue Ivy, 12, and seven-year-old twins Rumi and Sir with husband Jay-Z - said: "I'm not here as a celebrity. I'm not here as a politician. I'm here as a mother.

"A mother who cares, deeply, about the world my children and all of our children live in.

"A world where we have the freedom to control our bodies. A world where we're not divided.

"Our past, our present, our future merge to meet us here."

Willie had opened the event by asking those assembled: "Are we ready to say Madam President?"

He then sang some of his classic hits, including 'Mammas Don't Let Your Babies Grow Up to Be Cowboys' and 'On the Road Again'.