The vote for a new leader of Nicolas Sarkozy's rightwing UMP party turned into a shambles on Sunday night after both candidates declared themselves the winner amid a vicious public slanging match over dodgy ballots, trickery and fraud.

In the worst party political mayhem in recent French memory – even more savage than the famous in-fighting in the Socialist party – the right was left in paralysis as the different teams of the two self-declared leaders tore into each other on live TV. French media described the electoral chaos as "surreal".

More than 50% of the 300,000 members of the UMP party turned out to vote for a new leader on Sunday following Sarkozy's defeat in the presidential election in May.

During a long and bitter election campaign, Jean-Francois Copé, 48, a tough-talking hardliner, had played to the far-right audiences, saying that French city suburbs brimmed with "anti-white" racism and that the right must take to the streets to oppose the Socialist president, François Hollande.

He said: "I am the tenant of a right which does not have hang-ups, which tells the truth and is comfortable with itself."

Francois Fillon, 58, Sarkozy's former prime minister, depicted in the press as a mild-mannered provincial gentleman, styled himself as more moderate and conciliatory, promising to "unite" the party. Fillon repeatedly scored higher in popularity polls among the general public.

But the party was left confused and damaged when the voting booths closed and both camps immediately made accusations of ballot fraud, trickery and irregularities, lodging complaints with the party's internal election monitoring body.

The result was said to be too close to call and, with recounts and checks by the monitoring body likely, it became clear that the final result would not be announced until Monday at the earliest.

Amid this confusion, France was astonished to see Copé hastily take to the stage in front of live TV cameras and announce that he had won, in a rapidly delivered victory speech in which he vowed to work "hand-in-hand" with the loser.

Minutes later, an incensed and shocked-looking Fillon took to the stage at his head quarters to announce that he himself had won by a small margin, and that voters should now await the officially announcement of the results by the party's internal commission. "I will not let party members' victory be stolen from them," he said.

Key members of both camps rushed to TV studios and proceeded to savage each other live on camera, each insisting they had won.

The party's internal electoral commission had not yet pronounced on the final result or on the alleged voting irregularities.

The election night chaos threatens to hugely damage the party created 10 years ago by Jacques Chirac, and which had always been in power until, in recent months, it lost the Senate, then the presidential election, then the parliament elections.

Florian Philippot, of the far-right Front National, which has directly targeted UMP voters, described the election night as something "between Dallas and a Punch and Judy show".