Johannesburg, South Africa (CNN) -- Nelson Mandela turned 93 on Monday surrounded by family, as South Africans paid tribute to his legacy by performing at least 67 minutes of public service.

Sixty-seven is the number of years the former South African president devoted to public service.

Mandela -- affectionately known by his clan name Madiba -- was spending the day in Qunu, where he grew up.

South Africans held various activities to honor a man credited with helping bring together a nation separated by apartheid.

Early Monday about 12 million students sang a special "Happy Birthday" song in unison nationwide before they started their lessons.

A group called "Bikers for Mandela" rode around the country for eight days doing volunteer service, including painting playgrounds and planting trees. Their trip was scheduled to end in Pretoria on Mandela Day.

Read ways to keep Mandela's legacy alive

Mandela rarely makes public appearances, with his last sighting at the closing ceremonies of the World Cup in South Africa last year.

He was briefly hospitalized in January for an acute respiratory infection, and continues to receive medical care at home.

Under South Africa's apartheid regime, Mandela spent 27 years in prison after being convicted of sabotage and attempts to overthrow the government. He was released in 1990 and became president in 1994.

The United Nations joined his foundation in urging the world to perform 67 minutes of public service.

"Together, the best way we can thank Nelson Mandela for his work is by taking action for others and inspiring change," U.N. chief Ban Ki-moon said last week.

"Tutor a child. Feed someone less fortunate. Care for your environment. Volunteer at a hospital or community center. Be a part of a global movement to make the world a better place," Ban said.

U.S. President Barack Obama, whose wife, Michelle, met Mandela last month, said the icon is "a beacon for the global community" and for those who fight for justice.

"Madiba sets the standard for service worldwide, whether we are students, shopkeepers or farmers, Cabinet ministers or presidents," the president said in a statement. "He calls on us to serve our fellow human beings, and better our communities."

The United Nations designated July 18 as Mandela Day in 2009.