Johannesburg (CNN) – Presidents and prime ministers, celebrities and royals joined tens of thousands of South Africans to pay tribute to Nelson Mandela 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 France’s Francois Hollande gathered alongside street sweepers, actors and religious figures to pay tribute to the revered statesman who died last Thursday.

Despite the 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.

Some had skipped work and queued for hours to secure a seat so that they could pay their respects.

The four-hour service started an hour late as guests continued to stream into the stadium, where Mandela delivered his first major speech after his release from prison. It began with the national anthem.

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The stadium, which can seat around 90,000 people, slowly filled up as guests such as British Prime Minister David Cameron, United Nations Secretary General Ban Ki-moon, who will speak at the event, and Liberian President Ellen Johnson-Sirleaf arrived. Nobel Peace Prize laureate Archbishop Desmond Tutu and South African President Jacob Zuma were among the first to arrive at the stadium, as was former South African President F.W. de Klerk.

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 the famous faces arrived.

The world of entertainment also was well represented with South African actress Charlize Theron and U2′s Bono in attendance.

Mandela’s widow Graca Michal and his former wife Winnie Mandela embraced and kissed as they arrived.

With 91 heads of state attending, security was tight.

Authorities have been stepping up surveillance as presidents of six nations prepare to pay tribute to the late anti-apartheid leader in the service that will likely bring much of South Africa to a stop.

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.

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“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.

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.

U.S. official: South Africa experienced at hosting crowds

U.S. officials are satisfied with security arrangements.

“We have not heard any concerns,” Deputy National Security Adviser Ben Rhodes told reporters aboard Air Force One as President Barack Obama flew to South Africa.

“The South Africans hosted the World Cup, so they have experience hosting significant crowds and managing events like this, although clearly this is really a unique event in world history, given the number of leaders coming to pay their respects, as well as the people of South Africa.”

Given Mandela’s ailing health, the U.S. Secret Service made some arrangements in advance, a Secret Service spokesman said. But work that would usually take months to complete has been done in less than a week, the spokesman said.

“It’s a compressed timeline, but there are certain protocols we must have in place for any trip,” the spokesman said.

Those protocols involve securing the president’s motorcade route and hotel rooms and doing security walk-throughs.

The spokesman declined to offer specific details on security measures at the stadium.

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.

Presidents set to speak at service

Among the speakers at Tuesday’s memorial will be President Obama, who like Mandela was his nation’s first black president. Obama has cited Mandela as his own inspiration for entering politics.

In addition to Obama, former Presidents Jimmy Carter, George W. Bush and Bill Clinton will attend. More than two dozen U.S. lawmakers also plan to go.

Other guests include the Prince of Wales, British Prime Minister David Cameron as well as celebrities such as Oprah Winfrey and Naomi Campbell.

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.

In addition to Obama and Ban, the presidents of Brazil, Namibia, India, Cuba and South Africa are expected to speak at the service, as are family members, friends and others.

Monday events

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 names

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