South African police have intensified their search for suspects in the bombings of Soweto township on Wednesday.
Senior figures in South Africa blamed white right-wing extremists for the 10 bomb blasts and a number of whom have been arrested in recent months over alleged plans to overthrow the government.
Police said they were following up crucial leads about the most serious bombings since apartheid white rule ended in 1994, but stressed they still did not know who was responsible for the attacks and said no one had been arrested in the blasts.
"The investigation has been intensified. We are working very hard day and night," police spokesman Phuti Setati told journalists on Friday.
Nine explosions rocked Soweto township near Johannesburg, South Africa's financial hub, killing a woman and raising fears that white extremists were back on the warpath. A 10th explosion in a town near the capital Pretoria injured two people.
President Thabo Mbeki said in a regular online letter to supporters on Friday that such acts of terror were an attempt to undo the largely peaceful political transition South Africa has enjoyed.
"Bombs, terrorism and threats of violence...constitute an attempt to take away the miraculous achievement of 1994. The people of South Africa will unite to defeat those who want to return our country to a past it has rejected," he said.
Analysts say a small right-wing minority wants to provoke a racial backlash by the black-led government, believing this will push more whites to embrace their goal of a separate homeland.
Police said in September they had uncovered a plot to take over television stations, police stations, army bases and water supplies and to create this chaos in such a way that blacks were blamed.
Setati said police had not yet linked the bombings to their long-standing investigation into a larger plot.
However, senior government officials have said they believe a white right-wing group was responsible for the attacks, and President Mbeki pointed the finger at white racists in his response to the bombings on Wednesday.
In connection with the alleged coup plot, two white men, Jurie Johannes Vermeulen, aged 35, and Frederick Johannes Naude, 54, were charged in a Pretoria court on Friday with treason and terrorism and will join 15 other suspects due to be tried by the High Court next May, state prosecutors said.
Advocate Suzanne Auret said police were still hunting for six other plotters, including the alleged ringleader.
So far all those arrested have been from the white Afrikaner community descended from Dutch and French settlers who arrived at the southern tip of Africa 300 years ago.