'It was panic stations,' said a government delegate. 'The policemen standing guard at the door told us that if we heard shots we should all lie down on the floor. Joe Slovo (the Communist Party chairman) looked particularly anxious - quite rightly.'
Also locked inside the office were Roelf Meyer, a cabinet minister and the government's chief constitutional negotiator; Cyril Ramaphosa, secretary general of the ANC, and prominent figures in the Pan-Africanist Congress and the Inkatha Freedom Party, among dozens of others.
Outside, men in black SS-style uniforms, the 'Iron Guard' of the AWB (Afrikaner Resistance Movement), swarmed around the corridors of the World Trade Centre building with pistols and shot-guns looking, they said, 'for certain people'. However, no shots were fired.
The ANC President, Nelson Mandela, later called for the arrest of the mob's political leaders and threatened to revive the military wing of the ANC, Umkhonto We Sizwe, to protect negotiators. He said Mr F W de Klerk, the South African President, assured him the arrests would start from last night, and added: 'If it had been blacks, hundreds would have been killed on the spot.'
Events started at 8am with a demonstration outside a perimeter fence surrounding the building.
The organisers were the Afrikaner Volksfront, a far-right umbrella group headed by a retired chief of the South African Defence Force, General Constand Viljoen, in alliance with the leader of the parliamentary Conservative Party, Ferdi Hartzenberg.
A cry went up, the demonstrators surged forward and the police manning the main gate stepped gingerly aside as some 2,000 people, all of whom - it seemed - carried guns, swept into the compound.
The AWB leader Eugene Terreblanche headed the procession across 200 yards of open field to the building. Then, at about 9am, gave orders for the armoured vehicle in which he had arrived to drive into the main entrance's large glass doors. It smashed through, opening a wide breach through which hundreds poured.
Leaders of the various parties immediately took refuge in the small office. The initial panic - when it seemed Mr Meyer and Mr Ramaphosa might either be shot or held to ransom - passed when Mr Terreblanche, General Viljooen and Mr Hartzenberg walked through the breach and strode upstairs to talk to government officials.
Meanwhile, as riot policemen in camouflage uniforms looked on in confusion, the main negotiating chamber was overrun by men in black and khaki uniforms.
They daubed the chamber's walls with a slogan, One 'volk' one fatherland, lounged with their feet on the tables in seats normally used by delegates and journalists, shouted racist abuse at black staff members and an Indian delegate, and - reeking, some of them, of alcohol - spat on the floor.
At 11.15am the Afrikaner Volksfront leaders reached a deal with the government officials. They would leave the premises peacefully if no arrests were made. The officials agreed, but said they reserved the right to press criminal charges later.
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