John Travolta has led the tributes to Dame Olivia Newton-John, who died in Southern California aged 73 on Monday morning (local time) after a battle with breast cancer.
Travolta wrote an emotional post on Instagram, addressed to "my dearest Olivia".
"You made all of our lives so much better. Your impact was incredible. I love you so much.
"We will see you down the road and we will all be together again.
"Yours from the first moment I saw you and forever! Your Danny, your John!"
The pair starred alongside each other in the 1978 musical film Grease, singing major duets You're the One That I Want and Summer Nights. The film's soundtrack is one of the best-selling movie soundtracks of all time.
The British-born, Australian-raised singer and actress was a four-time Grammy Award winner and also campaigned extensively for environmental and animal rights issues.
American singer and actress Barbra Streisand wrote that her friend was "too young to leave this world".
"We have lost a great, iconic artist in Olivia Newton John, gone too soon from us at age 73," said John Takei.
"I trust she is now in the great Xanadu beyond. Know that we are forever hopelessly devoted to you, Olivia. Rest in song and mirth."
Long-time friend Richard Marx said his heart was broken.
"Rest now, sweet friend. You were as kind and loving a person as there’s ever been. I’ll miss you every day."
Kylie Minogue, whose career path followed that of Newton-John with her elevation to international fame from humble beginnings in Melbourne, said on Twitter: "Since I was 10 years old, I have loved and looked up to Olivia Newton-John. And, I always will.
"She was, and always will be, an inspiration to me in so many, many ways."
Hollywood actor Hugh Jackman described Newton-John as his "first crush".
"One of the great privileges of my life was getting to know her," he wrote on Instagram.
"Not only was she one of the most talented people I've known … she was one of the most open hearted, generous and funny. She was a one-of-a-kind spirit.
"I kissed her (poster) every night before bed. Her legacy will only grow stronger in the years to come. A fighter for healing from cancer that knows no bounds. I love you Olivia."
Singer Delta Goodrem, who played Newton-John in a 2018 biopic, wrote on Instagram that the star was both a mentor and friend.
"The whole world will feel this heartbreak today because the entire world felt Olivia's unmatched light," she said.
"I don't have all the words I would like to say today, but I hope everyone will join in celebrating our beloved Olivia, her heart, soul, talent, courage, grace ... I love you forever."