"Now after a lifetime of service, of giving, even stricken as he is in hospital, Madiba is uniting the nation again, this time in prayer," said Tutu, referring to his friend's clan name.
Citing the outpouring of wishes and prayers from all races, the retired archbishop said Mandela was still a bridge across the sometimes bitterly divided nation that he steered into multiracial democracy.
"Although our hearts are heavy, may we have the strength and the good sense to recognise this moment for what it is: an opportunity to reflect just once again on our promise to harness our collective energy," he added.
"We pray for his comfort and his dignity. We pray that we will be ready to commit him into the hands of our loving God," he said at the opening of an exhibition on Mandela in Cape Town.
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Tutu later met visiting President Barack Obama at his foundation's HIV Centre in the city, where he said it was a "special joy" to welcome the US leader to "the continent of his forebears".
There was also a lesson in the southern African "ubuntu" philosophy of humanity towards others -- with a hint of criticism about Guantanamo Bay.
"As you have been here before to Africa, you have heard us speak of something called ubuntu... Your success is our success. Your failure, whether you like it or not, is our failure. And so we want to show you that we pray for you to be a great success," he said.
Obama recently renewed his pledge to shutter Guantanamo, which was set up in the wake of the September 11, 2001 attacks to hold terror suspects captured on foreign battlefields.