BLOEMFONTEIN, South Africa - A splinter group of prominent politicians from the African National Congress launched a new party yesterday in the first major challenge to the governing party since it took power nearly 15 years ago after toppling South Africa's white-minority government.

The ANC and newly formed Congress of the People, or COPE, held dueling rallies.

The ANC's star power drew the usual large crowd. But the opposition politicians gathering across town delivered enough biting rhetoric to worry the nation's dominant political force.

While its ranks are filled with former ANC officials - though none with the charisma of current ANC leader Jacob Zuma or party icon Nelson Mandela - COPE is not expected to defeat the ANC in general elections next year.

But the ANC is worried there might be enough voter discontent for the new party to significantly cut into its big parliamentary majority.

Yesterday, the governing party showed how seriously it was taking COPE, organizing a counter rally in this university town 250 miles south of Johannesburg, which the ANC considers its birthplace.

Zuma, 66, a former guerrilla leader, drew thunderous cheers from 15,000 people packed into a soccer stadium in Mangaung, a black township on the edges of Bloemfontein. He called on supporters to "defend the ANC from attempts to sow disunity and confusion in its ranks."

COPE, led by former Defense Minister Mosiuoa Lekota, emerged after the ANC forced Thabo Mbeki to step down in September as the nation's president, culminating a power struggle with Zuma. Zuma had defeated Mbeki last year in a bitter race for the party leadership.

Lekota has called the manner in which Mbeki was ousted undemocratic and questioned whether Zuma, who has struggled to shake corruption allegations, was fit for the presidency.

Lekota said his party was formed because South Africa "was suffering from a crisis of leadership" and because of "concerns for the moral decay in our body politic."

Most of those at the new party's launch are Mbeki supporters, but the group does not have his public support.

The new party has been accused of offering little more than criticism of the ANC. Still, it has won a number of victories in recent local elections, while attracting whites and unveiling a more business-friendly economic policy to woo voters nervous about Zuma's close links to labor and the South African Communist Party.

The ANC won by a landslide in the last elections in 2004, securing more than a two-thirds majority in parliament, so it hardly faces the prospect of defeat anytime soon. Still, in the long term, many South Africans believe the breakup of the ANC is the only way to ensure their democracy matures.