By Joe Bavier

MATADI, Congo, March 19 (Reuters) - A Congolese national flag flies over the charred rubble of the former stronghold of a shadowy separatist sect in western Congo after a government offensive that killed dozens of its members.

In an operation U.N. investigators estimate killed at least 68 people since Feb. 28, police destroyed the headquarters in Democratic Republic of Congo's west Bas-Congo province of the ethnic-based religious and political sect Bundu dia Kongo (BDK).

"We are going to put a new police station right here," said deputy police commissioner Edmond Bunga, smiling as he gestured to the razed building that had served as BDK's spiritual base.

Truckloads of police in riot gear brought in from the Congolese capital Kinshasa, some with bayonets fixed to the barrels of their AK-47s, patrol the streets of Matadi.

The BDK, which authorities accuse of carrying out summary executions in the name of popular justice, seeks to re-establish the pre-colonial Kongo kingdom in parts of Democratic Republic of Congo, Republic of Congo, Angola and Gabon.

President Joseph Kabila's government has said the latest anti-BDK offensive is part of moves to restore state authority across the vast, mineral-rich former Belgian colony, which still suffers from the depredations of eastern rebels and militias.

The government is sticking by an initial death toll of 22 killed given by authorities. It rejects allegations by local witnesses of indiscriminate killings, abuses and torching of homes as police hunted down BDK sympathisers from town to town.

But the three-week crackdown in Bas-Congo comes a year after a similar anti-BDK offensive that killed over 100 people. It has embarrassed the stretched United Nations peacekeeping mission in Congo which was slow to send more peacekeepers to the province.

Preliminary internal reports by U.N. investigators seen by Reuters, but not yet officially made public, indicate the death toll from the latest police operation could be higher than 68.

Witnesses have told U.N. investigators that more than 100 bodies were dumped in a river in two separate locations.

MAGIC AND MACHETES

Neighbours living near the BDK spiritual base or "zikwa" in the Belvedere district of the Congo river port of Matadi say its destruction by security forces was swift and brutal.

"It lasted about 45 minutes, but it was a real massacre," said one neighbour, Maturin Wenaba.

"Then the police looted our houses. They stole our money, our televisions," he added.

Victims were treated for gunshot wounds at local hospitals. Very few BDK militants are believed to possess firearms. Medical workers fear more wounded could be hiding in the bush, afraid to seek treatment for fear of being arrested as BDK members.

But the sect was feared by many local residents, and local police, who had shied from confronting the movement's militia, the Makesa.

"They had magic. You could shoot at them, but you could not hit them. Then they'd come and cut off your head with a machete," said deputy police commissioner Bunga.

He and local residents said the group meted out its own violent justice.

Bunga, pointing to a concrete slab in the shattered remnants of the BDK base in Matadi, said the sect had a "torture chamber" in the building.

Nevertheless, some locals were wary about the idea of having a new police station built on top of the BDK stronghold.

"As long as the police don't bother us, I guess it's okay. But you know how it is. If you have a uniform, you threaten, you abuse, you steal," Wenaba said. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/ ) (Editing by Pascal Fletcher)

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