Williamson is one of eight former policemen applying for amnesty for the blast at the African National Congress' London headquarters on March 14, 1982.
Williamson told the TRC amnesty committee he understood the former South African government's policy to be that security and intelligence action would be taken against the enemy wherever it was found. He said this included the use of force to prevent violence against the South African state and its occupants.
Williamson described how the bomb was prepared and then taken to London and placed at the ANC headquarters. He said it had been impressed upon him by Brigadier Piet Goosen, who has since died, that deaths should be avoided in the attack, especially those of non-ANC people.
He said after placing the bomb, the rest of the team returned to South Africa but he flew to Brussels where he later heard on news bulletins that the ANC headquarters had been destroyed in a massive blast.
He said the aim of the blast had been to demoralise the ANC on the 70th annivesary of its existence and also to bring home to the UK the hazards of providing sanctuary to the ANC.
"We wanted them to realise the ANC's war with South Africa could spill out on to the streets of London," said Williamson.
Earlier former police commissioner General Johann Coetzee, who has also applied for amnesty for the London bombing, told the committee former police minister Louis le Grange told the media suggestions that the South African police was involved in the bombings were laughable.
Coetzee said Le Grange had told him earlier the bombing had been approved by the government as a whole but had not specificied whether the order had come from the Cabinet or from former state president PW Botha.