It was the civil rights issue that helped launch the political careers of Jesse Jackson, Ken Livingstone, Paul Boateng and an enigmatic black activist from Brixton known as Sparticus.
On a cold winter's day in November 1985, these rising stars of the anti-apartheid movement led a crowd of 20,000 demonstrators into Trafalgar Square where they called for the release of Nelson Mandela and the downfall of Margaret Thatcher.
What they didn't know was that among the crowd were plain-clothed officers from Special Branch who had come to assess the threat posed by this eclectic mix of political firebrands. Their observations were recorded in a confidential report released now, 22 years later, under the Freedom of Information Act.
In it, each speaker was sympathetically marked on the quality of the speech and the manner in which it was received.
The officers' report begins: "Jesse Jackson was warmly greeted and said that this important march would make the world aware of the vicious Apartheid regime. In the manner of the late Martin Luther King , he said that Apartheid would be defeated in South Africa in 1985. His speech was interspersed with calls of 'free Mandela' which was taken up the crowd." The speech was followed by Oliver Tambo, the former president of the African National Congress (ANC) who the police dismissed as "uninspired and disappointing".
When it came to Ken Livingstone, the then leader of the Greater London Council (GLC), the officers noted he had accused Mrs Thatcher and President Botha of being "racists".
Mr Boateng, now the British ambassador to South Africa, was recorded as describing Britain as the "mother and father of racism", for which he also blamed Mrs Thatcher.
When Neil Kinnock's wife, Glenys, took to the stage she was subjected to "heckling from the extreme left".
Simon Hughes, who had been elected as a Liberal Democrat MP two years earlier fared no better it seems. Special Branch officers remarked that he was "loudly booed".
Later on in the afternoon the officers' attention was drawn to a man introducing himself as a "Brixton personality called Sparticus" who had asked the audience to support the "black brothers and sisters in their fight against oppression in South Africa". Sparticus, whose identity is never disclosed, said it was only a matter of time before the revolution taking place in South Africa spread to the whole of Southern Africa and led to the creation of a new state called Azania "where all blacks would live freely".
Mr Boateng's wife, Janet, a former member of the GLC, called for Sparticus to be made president of Azania.
At this point the police report said the demonstration was still "vociferous but good humoured".
But by dusk the mood turned to violence, triggering running battles with the police who made 144 arrests.
In the report, released by the Home Office under the freedom of information laws, the police blamed the violence on militant groups from the Socialist Workers Party and a contingent of Rastafarians associated with the "Mangrove Restaurant" in Notting Hill, west London.
The report said: "Even before the arrival of the three marches [from Hyde Park, Brixton and Tower Hill] it had been clear to the senior officers in the square that there were fringe groups bent in creating disorder."
Throughout the apartheid era the Thatcher government remained isolated in its refusal to endorse economic sanctions against South Africa. This led to claims that trading interests were more important to the Conservative government than the treatment of black Africans.
The anti-apartheid movement, and in particular the 24-hour vigil outside the South African embassy in Trafalgar Square, continued to be a source of friction and political embarrassment throughout the 1980s.
So much so that in 1987 Geoffrey Howe, the Foreign Secretary, proposed new laws to clamp down on the protesters.
But according to the newly released files, the then Home Secretary Douglas Hurd refused to go along with Mr Howe's proposal to create a "cordon sanitaire" or exclusion zone around the South African embassy.Reuse content