(Updates with attempt to seize PJCC)

BASRA, Iraq, Aug 26 (Reuters) - Iraqi police thwarted an attempt by Shi'ite militiamen to occupy a joint command centre in the southern city of Basra from which British troops withdrew overnight, a police spokesman said on Sunday.

Britain, which has been responsible for security in Basra since it joined the United States in invading Iraq in 2003, has begun withdrawing this year and is expected to pull its forces out of their last base inside the city within two weeks.

Basra police spokesman Lieutenant-Colonel Abdul-Kareem al-Zaidi said militiamen had tried to invade the provisional joint coordination centre (PJCC) that British troops had shared with the Iraqi army, possibly to ransack it.

The situation was resolved peacefully after a delegation from the militia held talks with officials, Zaidi said.

A spokesman for Shi'ite cleric Moqtada al-Sadr in the holy city of Najaf, Ammar al-Saadi, said a group of Mehdi Army militiamen loyal to Sadr had gathered in front of the PJCC and chanted victory slogans before withdrawing peacefully.

The British military, which still has about 500 men at Basra Palace, its city centre base, said Iraqi security forces had not asked for help.

"We spoke to General Mohan and he assured us that the situation is under control. The PJCC is under the control of the Iraqi army," British military spokesman Major Mike Shearer said, referring to the Iraqi general in charge of security in Basra.

A successful invasion of the PJCC would have been deeply embarrassing for British forces, which are keen to show that the Iraqi security forces are improving and are close to taking full control of security in Basra province.

British bases in the southern provinces of Maysan and Muthanna were ransacked by Iraqis after the British handed them over to Iraqi security forces in 2006.

Basra, Iraq's second city and the hub of its southern oil fields, has witnessed a turf war between rival Shi'ite factions, vying for control of its vast oil wealth.

The British contingent in Iraq has been reduced from about 7,200 last year to about 5,500 now, most of them based at an airport outside Basra.

Britain says its troops are no longer needed in Basra because the mainly Shi'ite city has not been the scene of sectarian fighting, as has Baghdad further north.

But British troops in the city have come under increasing attack since reducing their numbers. Thirty-four British troops have died in Iraq since April, making it the deadliest period since 2003. (Additional reporting by Ross Colvin and Wisam Mohammed in Baghdad)

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