South Africa's largest opposition party, the Democratic Alliance (DA), and the Inkatha Freedom Party (IFP) have formed a coalition to fight next year's general election.
Buthelezi served in the first post-apartheid government
Tony Leon of the DA and Mangosuthu Buthelezi of the IFP held a joint rally in Soweto on Sunday, during which they said that they planned to clip the wings of the ruling African National Congress (ANC).
Mr Leon said that South Africans feared that their country was becoming a "shadow of democracy" that was dominated by one party.
They worry that the hope of a rainbow nation will be destroyed by greed, insecurity and by a new racism
Democratic Alliance leader
"The biggest problem in our continent and in much of the developing world is where there has been a one-party state," he told the BBC's Focus on Africa programme.
Mr Buthelezi, who is also the Home Affairs minister, told the programme that said the two groups would remain separate, but would work together to challenge the ANC.
"It isn't an all of a sudden thing," he said. "We have always shared ideals... We both believe in multi-party democracy."
The ANC, which won the 1999 general election with more than two-thirds of the vote, has been plagued with internal problems in recent months.
The party's deputy leader, Vice-President Jacob Zuma, recently had corruption charges dropped against him, while a former parliamentary chief whip, Tony Yengeni, was convicted for corruption in connection with the same multi-million dollar arms deal.
Mr Buthelezi said that the coalition would challenge the ANC on the creation of jobs, economic growth, poverty, crime and health.
"What we need is a leadership willing to draw from the strength
and commitment of the South African people," Mr Buthelezi said.
Mr Leon urged South Africans to register as voters for the 2004 elections.
Tony Leon says South Africa is becoming a 'shadow of a democracy'
"My appeal to the people of South Africa is simply this: look
around you, see the country that you live in, see the lives of your
neighbours and families, then feel enough to register and vote," Mr Leon said.
"They worry that the hope of a rainbow nation will be destroyed by greed, insecurity and by a new racism," he said.
The DA has recently pressurised the ANC over the corruption allegations made against the vice-president - with Mr Leon calling for Mr Zuma's resignation.
Mr Zuma, who is a long time friend of President Thabo Mbeki, is billed as his possible successor.
South Africa's public prosecutor decided to drop charges against the deputy president himself, saying there was not enough evidence to prosecute.