JOHANNESBURG (CNN) -- — JOHANNESBURG (CNN) -- Presidents and prime ministers, celebrities and royals joined tens of thousands of South Africans to pay tribute to Nelson Mandela on Tuesday, in a memorial service celebrating a man seen as a global symbol of reconciliation.
In what has been billed as one of the largest gatherings of global leaders in recent history, world leaders from President Barack Obama to Cuba's Raul Castro gathered alongside street sweepers, actors and religious figures to pay tribute to the revered statesman who died last Thursday, aged 95.
Despite the heavy rain, the atmosphere inside Johannesburg 's FNB stadium was celebratory, with people dancing, blowing "vuvuzela" plastic horns and singing songs from the anti-apartheid struggle.
Many carried banners honoring "Madiba," Mandela's traditional clan name, or his picture. Others were draped in materials covered with his face or the green, yellow, black, red and blue colors of the South African flag.
Some had skipped work and queued for hours to secure a seat so that they could pay their respects at the stadium where Mandela delivered his first major speech after his release from prison.
The four-hour service, coinciding with U.N. Human Rights Day, is the centerpiece of a week of mourning and was expected to bring much of South Africa to a stop.
It began with the national anthem before South Africa's presidents -- past and present -- were introduced. There was a loud cheer from the crowd for F.W. de Klerk, the last leader of white South Africa, who shared a Nobel Peace Prize with Mandela for helping to end apartheid.
The joyous cries died down as speeches from Mandela's family, friends as well as a fellow Robben Island prison inmate began. Anguished faces listened quietly as a sorrowful chant to "Tata Madiba" filled the air. "Tata" means "father" in Mandela's Xhosa tribe.
'The world has lost a mentor'
There were loud cheers for President Obama, who like Mandela was his nation's first black president. He has cited Mandela as his own inspiration for entering politics.
"To the people of South Africa -- people of every race and every walk of life -- the world thanks you for sharing Nelson Mandela with us," he said in his speech.
In addition, the presidents of Brazil, Namibia, India, Cuba and South Africa were also expected to speak at the service.
"South Africa has lost a hero, they have lost a father. The world has lost a beloved friend and mentor," United Nations Secretary-General Ban Ki-moon said to loud cheers.
"Nelson Mandela was more than one of the greatest leaders of our time, he was one of the greatest teachers. And he taught by example."
The stadium, which can seat around 90,000 people, was filled with guests such as British Prime Minister David Cameron, the Prince of Wales, French President Francois Hollande and Liberian President Ellen Johnson Sirleaf.
Nobel Peace Prize laureate Archbishop Desmond Tutu and South African President Jacob Zuma were among the first to arrive at the stadium.
Members of The Elders, a group of retired statesmen founded by Mandela and others were also in attendance, including former UN Secretary-General Kofi Annan and former U.S. President Jimmy Carter
The crowds cheered loudly and clapped as a huge screen showed famous faces, namely for the arrival of Obama.
The world of entertainment also was well represented, with South African actress Charlize Theron and U2's Bono in attendance. Celebrity guests also included Oprah Winfrey and Naomi Campbell.
Mandela's widow, Graca Machel, and his former wife Winnie Mandela embraced and kissed as they arrived.
Paying tribute to his uncle, General Thanduxolo Mandela gave thanks to the outpouring of respect from around the world.
"This universal show of unity is a true reflection of all that Madiba stood for -- peace, justice, unity of all mankind," he said.
"Let us pledge to keep Madiba's dream alive."
With 91 heads of state attending, security was tight.
Working off plans developed for years in secret, the South African government is using an elite military task force, sniper teams and canine teams to help secure the stadium, CNN's Arwa Damon reported Monday. In addition, helicopters and military jets frequently fly overhead.
"Should anybody, anything dare to disturb or disrupt this period of mourning and finally taking and accompanying the former president to his last resting place, then that person will be dealt with," Brig. Gen. Xolani Mabanga said Monday.
South African officials won't give details about their security plans -- how many police officers, how many troops, precautions to keep the stadium weapons- and explosives-free.
"But we can assure that all necessary steps have been taken, and that is why the leadership of the world and former leaders of the world have confidence to come to our country at this time to share with us this moment," said Minister in the Presidency Collins Chabane.
U.S. officials said they were satisfied with security arrangements.
The event promises to rival other significant state funerals in recent decades, such as that of former British Prime Minister Winston Churchill in 1965 and the 2008 funeral of Pope John Paul II, which attracted some 2 million people to Rome -- among them four kings, five queens, at least 70 presidents and prime ministers and the leaders of 14 other faiths.
At that event, metal detectors and some 15,000 members of security forces stood watch.
Security was also stepped up outside Mandela's home, where crowds showed up with umbrellas to show their appreciation of a man they said represented unity. Some even said they missed work for the occasion.
"We want to respect our father of the nation, our father of the country. That is why we left work to pay that respect to him," one South African told CNN.
Presidents set to speak at service
While Tuesday's memorial is the first major event honoring Mandela since his death, it won't be the last.
A state funeral will be held Sunday in Mandela's ancestral hometown of Qunu in South Africa's Eastern Cape province.
Crews worked overtime Monday to prepare stadium or the service.
The government has set up overflow locations at stadiums and other facilities throughout the country.
With private vehicles banned from the area around the stadium, the government pressed buses from around the country into service and stepped up train service to move the crowds.
In Soweto township, where Mandela lived before he was imprisoned for 27 years, people waited for three hours for buses to take them to the stadium. Unfazed by the wait, they sang and danced.
South Africa's Parliament reconvened Monday for an afternoon of speeches and memorials to Mandela. Dozens of members of parliament spoke.
"The world over, his name has evolved into a metaphor," Deputy President Kgalema Motlanthe said. "The name Nelson Mandela has entered the pantheon of history's sages."
Out of the public eye, friends who had not seen each other in years have been coming together with Mandela's family in his home, said Zelda la Grange, Mandela's longtime personal assistant.
Mandela called la Grange his "rock," even though she seemed an unlikely confidante. She was a white Afrikaner and an employee of the former apartheid government.
In her first interview since Mandela's death, she described the mood in his home to CNN's Robyn Curnow on Monday.
"Obviously there's sadness in the house," she said, but also, "People are celebrating Madiba's life. They are grateful." She referred to Mandela by his well-known clan name.