Johannesburg - The last and the biggest of four special auctions of ivory stockpiles got underway in South Africa Thursday after a nearly two-hour delay caused when Asian buyers refused to bid for 51 tonnes of tusks in front of the media.

Around two dozen mainly Chinese and Japanese buyers threatened to boycott the one-off sale unless journalists were barred from the conference centre of the Reserve Bank in Pretoria, where the auction opened shortly after 10 am (0800 GMT).

The ivory was not on show in Pretoria. Instead, images of the tusks, which are being stored in Kruger National Park, were being displayed on a big screen.

Lot number one had just appeared when the buyers informed SANParks that they wished to keep their identities confidential and demanded the around 30 journalists present, including teams from CNN and German television, be removed. SANParks, which is the national parks division of the department of environmental affairs and tourism, acceded to their demand.

The sale is one of four taking place nearly a decade after the last authorized sale of ivory in southern Africa in 1999.

The ivory trade has been banned since 1989 but the 171 members of the UN Convention on International Trade in Endangered Species (CITES) have given the go-ahead for four countries with thriving elephant populations to sell off ivory accumulated in their national parks.

The one-day auctions kicked off in Namibia on October 28, before moving to Botswana, Zimbabwe and Pretoria, where Thursday's sale was taking place at the South African Reserve Bank. The four countries have a combined population of 312,000 elephants, the South African government said Thursday. The other three auctions also took place behind closed doors, despite media objections.

In total, 108 tonnes of ivory, harvested mainly from elephants that died accidentally or of natural causes in national parks, went on the auction block. Of the 51 tonnes for sale in Pretoria, 46 tonnes comes from the world-famous Kruger National Park.

Of this stock, local media reported that some was from elephants culled before 1994, when the practice was allowed to control the pachyderm population. South Africa only last year relaxed a 13-year moratorium on the practise, allowing culling as a measure of last resort.

Animal rights activists have opposed the auctions, arguing that all sales of ivory - even legal - stimulate the black market trade in the product and, consequentially, elephant poaching.

After Thursday's sale is over, the trade will have to wait at least another nine years before CITES says it can envisage another sale. (dpa)