LUSAKA (Reuters) - Zambia was set to declare the winner of cliffhanger presidential elections on Tuesday, while opposition leaders prepared for a mass protest against alleged ballot-rigging.

Certified results from the Electoral Commission late on Monday showed Levy Mwanawasa, President Frederick Chiluba's chosen heir, extending his narrow lead over Anderson Mazoka of the opposition United Party for National Development (UPND).

But leaders from Zambia's ten opposition parties rejected the results and urged the former British colony's 11 million citizens to demonstrate against the outcome, which was also criticized by independent observers.

``We in FDD (Forum for Democracy and Development) join other opposition leaders in calling upon our members countrywide to start peaceful protests tomorrow against the fraudulent results,'' FDD leader Christon Tembo told a news conference.

Tembo was running third in the poll after Mazoka, a 58-year-old businessman who has declared himself the winner and accused Chiluba of trying to cheat him of victory.

Protests over the outcome on Monday have heightened political tension in Zambia, seen as one of Africa's strongest democracies and usually an oasis of peace in the conflict-torn continent.

But Chiluba and the MMD are being blamed for corruption, economic mismanagement and reviving divisive tribalism.