The participants, some in South African costume, handed out South African oranges before going to Downing Street to hand in a petition.
Ten years ago, the demonstrators would have been from the Anti-Apartheid Movement, trying to shame Britons into boycotting Outspan oranges. Now it is the movement's successor, Action for Southern Africa (Actsa), trying to shame the European Union into dropping its attempts to block exports of South African produce.
"It's ironic," said Actsa's director, Ben Jackson, "but trade will be the biggest issue during President Mandela's visit."
The symbolism of Mr Mandela's state visit to Britain is immense: today, apartheid's most prominent political prisoner, once reviled by British government leaders, will be received with 21-gun salutes and will ride in a state carriage down the Mall with the Queen. But behind the scenes in both Britain and France, where Mr Mandela will make another state visit, he will be demanding that the EU honour its promises to help the whole of southern Africa to overcome the economic aftermath of apartheid.
Two years after the euphoria of South Africa's first free elections, the EU has failed to agree on a proposal, backed by Britain, for the creation of a free trade area with South Africa.
Despite a growing trade surplus with South Africa, Germany is leading efforts to exclude nearly 40 per cent of South African farm exports - including oranges, tinned fruit and wine - from talks on better market access.
Actsa argues that the EU, South Africa's most important export market, treats its products little differently from when it was an international pariah.
It was easier to rally the British public when Mr Mandela was imprisoned and apartheid was at its worst.
Membership of the Anti-Apartheid Movement (AAM) peaked at around 25,000 to 30,000 in the late Eighties, when there was a permanent vigil on the pavement outside South Africa House.
The giants of rock music fell over themselves to take part in the Mandela birthday concert at Wembley in 1988, bringing an influx of younger people.
The vigil came to an end in February 1990, when Mr Mandela walked out of prison after 27 years. In October 1994 the AAM, its work done, disbanded after 35 years and transformed itself into Actsa, which now has about 5,000 members.
"There has been a lot of continuity," Mr Jackson said.
"Trade unions, which were always very active in the AAM, have been very good about reaffiliating to Actsa, which was not an automatic process."
The AAM's headquarters in north London, which suffered a firebomb attack by South African agents in the early Eighties, are now closed.
Actsa took over the nearby offices of the African National Congress following the ANC's transformation from an exile guerrilla movement into South Africa's governing party.
Actsa's patron is Bishop Trevor Huddleston, who founded the AAM in 1959 with Julius Nyerere, then President of Tanzania.
South Africa has given Actsa a grant of pounds 80,000 to catalogue the AAM's archives, which will be housed in the Bodleian Library, Oxford.
Is there still a warehouse full of "Free Mandela" T-shirts somewhere?
"There is some merchandise left over, which we sell off from time to time to raise money," Mr Jackson said with a laugh.
"We had some former freedom fighters from Namibia over who are now MPs. One of them asked me for a 'Free Mandela' mug to replace one he had broken. I gave him two."Reuse content