By Gray Phombeah BBC reporter in Nairobi

A carnival mood swept the Kenyan capital Nairobi on Monday, with a riot of colours and sounds as two main candidates officially launched their campaign to succeed President Daniel arap Moi in the December polls.

Tens of thousands of Kenyans marched through the city in the scorching afternoon sun - chanting, whistling and waving banners.

Businesses closed down and workers stayed away for fear of violent clashes between supporters of the ruling Kanu party's candidate Uhuru Kenyatta and those of the main opposition's flagbearer, Mwai Kibaki.

We must fight corruption which has made Kenya a pariah nation

Mwai Kibaki, National Rainbow Coalition

Trucks decorated with party posters drove through the city, carrying dancers and singers praising their respective candidates and pouring scorn on their opponents, through mounted sound systems.

In a striking departure from the past, riot police carrying rifles, batons and tear gas canisters simply kept the two sides apart and watched the unfolding political drama.

Huge crowds

President Moi - who has been in power since 1978 and is obliged by the constitution to step down - was reduced to a side-show as he accompanied his hand-picked successor to the electoral commission at Nairobi City Hall to present nomination papers.

Later he sat pensively as Mr Kenyatta addressed a crowd of nearly 50,000 supporters - most of them probably bussed in from nearby districts - at a Kanu rally.

Mr Kenyatta - aged 42 and the son of Kenya's first president, Jomo Kenyatta - spoke of the new generation in Kenya. It was a clear dig at Mwai Kibaki, who is 71 and a former vice-president in Mr Moi's government.

Kenyatta seeks to continue the ruling party's reign

"This is an opportunity that has been given to a new generation to bring fundamental changes in this country... bring new ideas in the leadership of this country," he said.

Mr Kibaki, flanked by the opposition leaders united by hatred for Mr Moi, presented his papers to the beat of drums and traditional dancing at City Hall.

Mr Kibaki also addressed a huge crowd of his supporters a few kilometres from Mr Uhuru's rally - winning the battle for numbers by pulling a crowd of more than 60,000.

He told the cheering crowd that he would root out corruption, offer free education in primary schools, and restore the rule of law.

"We must fight corruption which has made Kenya a pariah nation. Internationally Kenya is known as a corrupt nation, which has scared foreign investors."

Close contest

Kanu faces its stiffest challenge from a united opposition after a 40-year grip on power since independence in 1963.

A coalition of 13 opposition parties has rallied behind Mr Kibaki, who is campaigning for change.

Kanu has suffered a string of defections by its senior leaders following Mr Moi's choice of Uhuru Kenyatta as his successor.

As night fell and supporters trekked back home, Nairobi's city centre became almost deserted - a brief lull before the two candidates hit the campaign trail on Tuesday in the final race to the 27 December polls.