President Daniel Arap Moi's overarching dominance of Kenyan politics abruptly ended yesterday as his ruling KANU party split in two and a powerful opposition coalition emerged to challenge his 24-year rule.
At least 80,000 opposition supporters thronged the city centre Uhuru Park for one of the largest gatherings since the advent of multi-party democracy in 1992.
Drawn from across Kenya's notoriously fractious tribal spectrum, the crowd sang, danced and interrupted speeches with the chant "Without Moi, everything is possible".
At the rally many former government ministers and their former opposition rivals announced they were forming a "super alliance" to ensure that President Moi and his inner circle are unseated at presidential elections promised for December.
The KANU rebels - which include the former vice-president Mr George Saitoti and five former ministers - were led by former secretary general Mr Raila Odinga.
After years of tribal division under President Moi, Kenya's opposition was finally coming together, he said. "We are making history here in Uhuru Park," he said. "And we will be in government next January." The KANU rebels announced they were setting up a new opposition party, the Liberal Democratic Party (LDP).
For many years seen as an impregnable election juggernaut under President Moi, the ruling Kenya African National Union party has never appeared more fragile.
The internal rebellion was sparked by President Moi's choice of Mr Uhuru Kenyatta - son of independence leader Jomo Kenyatta - to succeed him at elections scheduled for next December.
The move aroused widespread suspicious that Mr Kenyatta, who has little political experience, could be a stooge for the interests of Kenya's notoriously corrupt elite after President Moi's retirement.
Yesterday over 4,000 KANU delegates met in a sports hall on the other side of Nairobi to confirm Mr Kenyatta (41), as their candidate. In his acceptance speech, Mr Kenyatta vowed to "serve the nation with devotion".
Meanwhile, the opposition, for years divided by tribal rivalries and personal ambitions, vowed to unite behind a single candidate.
"Yes, all of us would have been interested in being president of Kenya," admitted opposition leader Mrs Charity Ngilu to the crowd. "But now we are working as a team. The positions we get will not matter." One senior alliance member told The Irish Times that Mr Mwai Kibaki would be confirmed as the agreed candidate at a press conference this morning.
Mr Kibaki is the leader of the Democratic Party and a member of Kenya's largest tribe, the Kikuyu.
The Uhuru Park rally was marked by a palpable sense of anger with the widespread corruption and economic failures that have characterised President Moi's rule.
One man carried a placard that read: "S Milosevic - Some Kenyans are joining you in The Hague soon."
"You see today is a working day and we are supposed to be employed. But look all the people that are here," said the unemployed Mr James Mwangi, before the rally.
On the grass beside him sat some of the many glue-sniffing street-children that roam Nairobi.
Many spectators admitted that some of prominent figures in the new alliance also presided over the current decline by being part of government.
"Yes, it is true," said one nurse who declined to be named. "But we have come here for a change. That is the most important thing now."
Despite earlier fears, there were no clashes between government and opposition supporters.