Volunteers carried out a wide variety of tasks including feeding street children, cleaning schools and caring for the elderly.
A group of 30 bikers including Hollywood actor Morgan Freeman took to the road from Johannesburg to Cape Town, engaging in community service along the way.
The group built a security fence at a centre for people living with Aids in the impoverished Khayelitsha township, outside Cape Town.
"I take it as my connection to Madiba [Mandela]. We should do it everyday," Freeman told journalists.
Activities were organised across the globe, from Sudan's Darfur region where participants in a "Football for Peace" tournament competed for the Nelson Mandela cup to Spain where a 6.7km charity walk was being held.
"Today is an opportunity for millions of people around the world to look inside themselves and find those beautiful qualities as any human being has and say: 'I am able to make a difference to my neighbor, to someone underprivileged, I can extend my goodness to other people,'" Graca Machel, Mandela's wife, said.
Jacob Zuma, South Africa's president, and other government officials were marking the day in Mandela's birthplace of Mvezo by planting trees and painting classrooms.
In a speech, Zuma said Mandela taught South Africans that "we must work together to entrench African unity and solidarity in our country".
"Of all the things that Madiba [Mandela] cherishes up to this day, it is the love of all humanity, freedom, justice and compassion for all people," he said.
Mandela was jailed for 27 years by the country's white minority government for resisting apartheid rule.
On his release in 1990, he led negotiations with apartheid rulers, a process that culminated in his election as the country's first black president in 1994.
Mandela stepped down as president in 1999, after serving one term in office and is still revered around the world for promoting peace and fighting against racism and HIV/Aids.
"Nelson Mandela is a towering figure. He embodies the highest values of humanity, and of the United Nations," Ban Ki-moon, the UN secretary-general, said in a statement on Sunday.
"Nelson Mandela's accomplishments came at great personal cost to himself and his family. Today, on the first Nelson Mandela International Day, we thank him for everything he has done for freedom, for justice and for democracy."
South Africa celebrated the 20th anniversary of Mandela's release from prison on February 11, a day credited with shaping the history of the country.