The retired actress and former U.S. First Lady died of heart failure, aged 94, and celebrity friends and fans wasted no time in expressing their sorrow on social media.

Action man Arnold Schwarzenegger, who served as Governor of California for eight years from 2003, 36 years after Nancy's late husband, President Ronald Reagan, held the same post, was among the first to take to Twitter to share his condolences for the former First family.

"Nancy Reagan was one of my heroes," he began. "She served as First Lady with unbelievable power, class and grace and left her mark on the world. She's with her Ronnie now, but those of us she left behind will miss her dearly."

Veteran screen star Joan Collins also honoured her memory online, writing, "My friend Nancy Reagan died aged 94 The end of an era, #Love the 80s (sic)", while Star Trek icon Shatner simply posted, "Condolences going out to the family of Nancy Reagan."

Elizabeth Banks recalled a previous meeting with the stylish former First Lady, who left her in awe.

"I sat near #Nancy Reagan once and felt like a teenager seeing one of my idols," The Hunger Games beauty shared. "She was a BOSS. #RIPNancy".

Fellow actress Melissa Joan Hart, who worked with Nancy at numerous events for the Ronald Reagan Presidential Library, also praised her as "one of America's great women", and Kristin Chenoweth tweeted, "sad day for us all, the embodiment of class and grace #nancyreagan".

Tributes also poured in from Vanessa Williams, Bette Midler, and Melissa Gilbert, who is friends with Nancy's actress/author daughter Patti Davis, while current U.S. President Barack Obama and his wife Michelle released a statement, revealing they "remain grateful for Nancy Reagan's life, thankful for her guidance, and prayerful that she and her beloved husband are together again."

Meanwhile, actor/musician John Stamos, who occasionally plays the drums for The Beach Boys, revealed the bandmates would honour Nancy's memory at their Sunday night show in Abilene, Texas.

"@TheBeachBoys will dedicate 'Their Hearts Were Full of Spring' tonight in loving memory of Nancy Reagan," he posted.