By Christian Tsoumou

BRAZZAVILLE, June 20 (Reuters) - Congo Republic's opposition demanded the replacement of the electoral commission with an independent body on Saturday and said next month's presidential election could not go ahead if it was not changed.

The opposition accuses the National Electoral Commission of being too close to President Denis Sassou-Nguesso, a former military ruler who lost a 1992 vote, took back power in a 1997 civil war and won a 2002 ballot that rivals branded a sham.

Four opposition presidential candidates were barred on Friday from contesting the July 12 presidential election in the oil-producing country of just over 4 million, which stretches back from the Atlantic coast into central Africa's forests.

At least 10,000 people joined an opposition rally in the capital, Brazzaville, on Saturday.

"We want a truly independent electoral commission, one that will carry out a proper voter census and distribute voters' cards," Mathias Dzon, a former finance minister and the main opposition candidate, told the crowd.

"We will fight to the end to obtain these demands and if we don't get them, neither Sassou, nor us, nor anyone else will take part in this election."

There was no immediate comment from the electoral commission or the authorities. The commission is appointed by the president.

Former prime minister Ange Edouard Poungui, who was among those barred by the constitutional court from contesting, told the crowd the ban was politically motivated.

He was forbidden because he had not shown he had lived in Congo for an uninterrupted period of two years but had used a foreign address during that time.

Sassou's main rivals were either banned from the 2002 ballot or withdrew. He won a landslide. (Editing by Matthew Tostevin)

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