"We need you!" one sign read. "We love you tata, get well soon!" said another.
Police also cordoned off the street outside Mandela's Johannesburg home Thursday, the South African Press Association reported.
About 30 journalists who were gathered outside were asked to move to the end of the street, but well-wishers were still being allowed to leave flowers and tributes by the house.
'A hero for the world'
Mandela became an international figure while enduring 27 years in prison for fighting apartheid, the country's system of racial segregation. He was elected the nation's first black president in 1994, four years after he was freed.
Even as he has faded from the spotlight, he remains popular and is considered a hero of democracy worldwide.
He turns 95 next month.
U.S. President Barack Obama, who is on a tour of the continent this week that includes South Africa, said his thoughts are with the nation's citizens.
"He is a personal hero, but I'm not unique in that regard," Obama told CNN Chief White House correspondent Jessica Yellin in Dakar, Senegal, the first stop of his African tour.
"I think he's a hero for the world, and if/when he passes, we know his legacy will linger on throughout the ages."
First lady Michelle Obama said Mandela "is very much in our thoughts and prayers right now." Speaking to middle-school students in Dakar, she urged the children to "honor his legacy by leaving a proud legacy of your own."
"If President Mandela could hold tight to his vision for his country's future during the 27 years he spent in prison, then surely you all can hold tight to your hopes for your own future," she said.
"If President Mandela could endure being confined to a tiny cell, being forced to perform back-breaking labor, being separated from the people he loved most in the world, then surely all of us, we can keep showing up and doing our best -- showing up for school each day, studying as hard as you possibly can."
Obama's schedule does not include a visit with the anti-apartheid icon.The president's trip to South Africa this weekend includes a stop on Robben Island, where Mandela spent a majority of his prison term.
After South Africa, Obama plans to head to Tanzania, his last stop before returning to Washington.