With more than three-quarters of the vote counted, the "yes" camp - backed by the president, Mwai Kibaki, was trailing opponents of the law by more than 1m votes.

"We have blown the trumpet and sent for the suits and champagne," Tom Kajwang, a senior official from the "no" team, said. Supporters of the "no" campaign celebrated, dancing and singing in the streets of Nairobi.

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However, the final text of the document split the ruling coalition, with a quarter of Mr Kibaki's 28-member cabinet opposing the policy.

The Liberal Democratic party, which holds a pivotal minority of seats on the government benches, has complained of being sidelined.

LDP supporters cried foul after plans to enhance the role of prime minister and parliament were dropped from the final text of the referendum.

The 153-page document also includes provision on everything from bans on abortion and same-sex marriage to equal rights for women, quotas for people with disabilities in public life, and the design of Kenya's flag.

"The 'no' vote does not mean that Kenyans do not want a new constitution," Patrick Lumumba, the secretary of the independent constitutional review commission, said.

"It means that they did not want what had been proposed in the current draft".

But the result is likely to transform the political landscape in Kenya ahead of its 2007 elections, adding to perceptions that Mr Kibaki has become a lame duck.

The popular enthusiasm that greeted his election in 2002 has quickly waned, as promises to reduce corruption and end tribal politics have proved hard to fulfil and the ruling coalition has become increasingly fractious.

Muzong Kodi, an Africa expert at the Chatham House thinktank, said he vote was a barometer of public opinion on the administration.

"This is not only a vote on the referendum, it appears also to be a vote of no confidence in the performance of Mr Kibaki's government," he said.

In a country where up to one third of the adult population is illiterate, ballot papers and political debate were simplified by representing the "yes" campaign with a banana and the "no" with an orange.

Reform to the political system in the former British colony is still seen by most as essential.

Kenya endured more than two decades of strongarm leadership under its former president Daniel arap Moi until 2002.

Mutula Kilonzo, the shadow attorney-general for Mr Arap Moi's Kanu faction and a leading member of the "no" camp, said Mr Kibaki's attorney-general and justice minister should resign over their roles in drawing up the new constitution.

Electoral officials reported a largely clean vote from Kenya's 19,000 polling stations, despite reports of vote buying, registration problems and other electoral irregularities in some districts.

At least seven people were killed in often violent rallies and clashes leading up to yesterday's vote.

As tensions rose over the count yesterday, hundreds of people chased riot police from Nairobi's vast Kibera slum, a stronghold of a leading opponent of the constitution.