They tuned in to watch performances by some of the biggest rock and pop stars of the day -- Dire Straits, Stevie Wonder, Whitney Houston, Simple Minds and Sting.
But for many of the half a billion television viewers who watched the 1988 Wembley Stadium concert, it was an eye-opening event that helped turn Nelson Mandela into a global icon.
"It was a game-changer. For the first time people began to realise who Mandela was," said Bob Hughes, the former chairman of Britain's Anti-Apartheid Movement (AAM).
"On the back of that, we carried out the campaigning and he began to be known as a person of conscience."
The Wembley crowd of 70,000 was dwarfed by the 250,000 who demonstrated in Hyde Park for Mandela's 70th birthday a few weeks later, and the 200,000 who attended a free anti-apartheid gig in London two years earlier, in 1986.
But as a televised event, the concert on June 11, 1988, provided a global platform to highlight the plight of Mandela -- then languishing in jail -- and other political prisoners in apartheid-era South Africa.
"We were breaking out from the street demos to a mass audience at home watching TV," said Jerry Dammers, a founder of ska band The Specials who helped organise the concert.
"They weren't necessarily as committed, but the sheer numbers -- that was the most important thing," he told AFP.
Within 20 months Mandela was free and just a few weeks later made a trip to London for a second concert at Wembley, where he thanked the crowd for the 1988 tribute.
"The concert was absolutely crucial in terms of raising Mandela's profile," said Saul Dubow, professor of African history at Queen Mary University of London.
"After the Wembley concert, there was a huge groundswell of popular support for Mandela. He became an icon of freedom -- even though very little was known about him."
'Don't call this man a terrorist'
The concert was originally intended as a fundraiser but as the lineup grew -- 83 artists would eventually take part -- the organisers began to realise the power they held in their hands.
The BBC agreed to film the event, paving the way for other broadcasters to sign up, covering almost 70 countries.
To win over the TV advertisers the concert was pitched as an entertainment event and the political message was toned down.
It is easy to forget now that at the time, international news media often referred to Mandela as a "jailed terrorist leader".
But the producer of the concert, Tony Hollingsworth, recalled how it prompted news editors to take a different line.
TV executives would call their newsdesks and say "don't call this man a terrorist, we just signed 11 hours of broadcasting for a tribute about him," he said.
"This is how we turned Mandela from a black terrorist into a black leader."
In fact, the artists performed in front of a banner reading "Artists Against Apartheid" and those attending were in no doubt what it was all about.
One concert-goer, Ben Padley, was only 12 at the time and admits the main draw for him was his favourite band the Eurythmics.
But he recalls: "I had a feeling of being part of something that was really important. And it formed some of my early thoughts about politics, how wrong this all was."
For Suresh Kamath, vice-chairman of the anti-apartheid movement, the magical moment was when The Specials played their 1984 hit anthem "Free Nelson Mandela".
"The atmosphere in the stadium was electric -- 70,000 people singing 'Free Mandela', and that was broadcast to hundreds of millions of people around the world," he told AFP.
'We were involved in a war'
The concert led to a huge surge in applications to join the anti-apartheid movement, and membership between 1985 and 1988 increased tenfold to about 30,000.
But Kamath stressed that the event should be seen in the context of years of grassroots campaigning against apartheid, as well as the shifting international situation.
By 1988, many countries had imposed trade sanctions on South Africa -- although not Britain -- and negotiations were gaining pace behind the scenes to get Mandela out of jail.
"You cannot just have a concert in isolation -- you have to have a campaign behind it. Persuading the artists to take part, you have to have credibility," Kamath said.
And it was no easy task to pull off. A number of British lawmakers tried to stop the BBC from broadcasting the concert, while the organisers received bomb threats.
Elsewhere, anti-apartheid activists were constantly under threat. Just a few weeks before Wembley, the Paris representative of the ANC was shot and killed in her office.
"They were great moments in my life, but behind the scenes it was quite unpleasant. We were involved in a war with a very evil thing," Dammers recalled.