The former South African president was taken to a medical facility after suffering a recurrence of a lung infection he previously battled in December (12).
The country's President Jacob Zuma asked well-wishers to pray for the former leader, and several celebrities have now taken to their Twitter.com pages to reach out to Mandela.
In a post, Groban writes, "Sending love to Nelson Mandela...back in the hospital. I hope he feels better soon. The world needs him as long as we can have him."
Halliwell, who is partway through the ex-leader's autobiography Long Walk To Freedom, adds, "Thoughts for Mandela, I'm currently reading his Long Walk To Freedom, grt (great) read. Inspirational."
Actress Holly Robinson Peete re-tweeted an article detailing Mandela's health scare, and urged the 94 year old to get well soon.
An update on Mandela's condition has come from his spokesman, MAC Maharaj, who tells Sky News, "At the moment doctors are saying it is a recurrence of an old lung infection. It is a matter of concern... They are doing everything they can to keep him comfortable and happy."