As an ailing Nelson Mandela recuperates in a South African hospital, the world celebrated his 95th birthday Thursday, honoring his legacy in various ways, including performing 67 minutes of community service.

Schoolchildren sang "Happy Birthday" to the former president during morning assemblies while crowds left flowers and candles outside his Pretoria hospital. The day also marks 15 years since he married his wife, Graca Machel.

President Jacob Zuma said that Mandela, who has been hospitalized with a lung ailment since June, is "steadily improving." Zuma's latest assessment comes after weeks of describing Mandela's heath as critical but stable.

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The president wished Mandela a happy birthday.

"We are proud to call this international icon our own as South Africans and wish him good health," Zuma said. "We thank all our people for supporting Madiba throughout the hospitalization with undying love and compassion. We also thank all for responding to the call to give Madiba the biggest birthday celebration ever this year."

South Africans affectionately refer to Mandela as Madiba, his clan name.

The United Nations declared July 18 as Mandela Day four years ago to honor his role in reconciling a country torn apart by apartheid. It started as a call to promote global peace and encourage community service.

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His foundation is asking people to volunteer 67 minutes of public service on his birthday, a reference to the number of years he devoted to public service.

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U.S. President Barack Obama, who visited with Mandela's family in South Africa last month, also sent birthday wishes.

"People everywhere have the opportunity to honor Madiba through individual and collective acts of service," he said in a prepared statement. "Through our own lives, by heeding his example, we can honor the man who showed his own people -- and the world -- the path to justice, equality and freedom."

The frail icon has not appeared in public for years, but he retains his popularity as the father of democracy and emblem of the nation's fight against apartheid.

His defiance of white minority rule focused the world's attention on apartheid, the legalized racial segregation enforced by the South African government until 1994.

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His hospitalization has given his birthday a sentimental touch. The South African Embassy in the United States said it will be the biggest celebration since his birthday in 1990, the year he was freed from prison.

The festivities are not limited to South Africa. In the United States, the embassy said 18 cities, including the nation's capital, will hold various events to celebrate his birthday.

The United Nations will also host an event in New York City featuring various speakers, including former U.S. President Bill Clinton, whose presidency coincided with Mandela's. The Rev. Jesse Jackson, the U.S. civil rights leader, and Andrew Mlangeni, 87, who was imprisoned with Mandela, will be part of the celebration.