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By David Lewis

KINSHASA, July 11 (Reuters) - Hundreds of Congolese demonstrators clashed with police in Kinshasa on Tuesday, demanding the destruction of millions of spare ballot papers printed for historic elections later this month.

A Reuters reporter saw riot police beat some protesters with batons and also fire stun grenades which knocked several people unconscious. Congolese Red Cross officials carried at least two people away.

At least 10 people were injured, some with head wounds, as officers firing tear gas chased demonstrators down sidestreets off the capital's main boulevard, witnesses said.

Demonstrators distributed pamphlets in the name of 19 presidential candidates who last week demanded the postponement of the July 30 polls, which will be the first free elections in 40 years in Democratic Republic of Congo.

The candidates demanded that 5 million spare ballot papers be destroyed to prevent them being used to fix the poll results. The electoral commission says it needs the spare ballots in case any others are destroyed by accident.

"Too many ballot papers have been printed. The population demands that they be destroyed," said one male demonstrator who declined to give his name, as police chased away protesters talking to foreign journalists.

The interior ministry has said it will allow public protests -- permitted under a new constitution overwhelmingly approved by a referendum last December -- if it receives three days notice.

Police said Tuesday's demonstration was prohibited, but protesters contested this.

"This is legal. We alerted the authorities. All we are asking for is free elections," said Franck Diongo, a member of parliament who helped organise the march.

The protest brought the heart of the city to a halt as police skirmished with groups of demonstrators before hundreds of officers launched a concerted operation to break up the demonstration using batons and tear gas.

Protesters, who included some supporters of the prominent UDPS opposition party which is boycotting the elections, tore down election campaign posters and chanted slogans against President Joseph Kabila.

Opponents say Kabila is favoured by foreign governments overseeing the landmark elections, which are designed to draw a line under Congo's 1998-2003 war and subsequent militia violence that have together killed an estimated 4 million people.

"Congolese brothers, open your eyes and rise up. They want rigged elections to make sure their candidate wins," said the pamphlets distributed by demonstrators, some of whom also shouted insults at foreign reporters, accusing them too of bias.