The ceremonies took place in the lovely sandstone amphitheatre formed by the Union Buildings in Pretoria. For decades this had been the seat of white supremacy, and now it was the site of a rainbow gathering of different colours and nations for the installation of South Africa’s first democratic, non-racial government. 

Explanation:

 

The ceremonies were held in the Union Buildings. The buildings are considered as the official seat of the South African government, and at the same time, they also house the offices of the South African President. These majestic structures are in Pretoria, one of South Africa’s three capital cities.  Cape Town is the legislative capital whereas Bloemfontein is the judicial capital.

 

The Union Buildings form a kind of amphitheatre, which was where the ceremony had taken place Since the buildings were made up of sandstone, Mandela calls the space the "lovely sandstone amphitheatre". 

 

Mandela adds that the Union Buildings had been the centre of white dominance for decades. However, the ceremony had instead made it the location of a rainbow gathering of people of all colours and nationalities to form South Africa's first democratic, non-racial government.

 

Meanings of difficult words:
 
Sl. No
Words
Meanings
1
ValleyA low area of land between hills or mountains, typically with a river or stream flowing through it
2
CrestThe top of a mountain or hill
3
RipeHaving arrived at the fitting stage or time for a particular action or purpose
4
DownpourA heavy fall of rain
5
ShowerA brief and usually light fall of rain, hail, sleet, or snow
6
IntimatelyIn a way that involves detailed knowledge
Reference:

National Council of Educational Research and Training (2007). First Flight. Nelson Mandela: A Long Walk to Freedom - Nelson Rolihlahla Mandela (pp. 16 - 23). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi..