By Ahmed Rasheed and Waleed Ibrahim

BAGHDAD, March 24 (Reuters) - Members of Shi'ite cleric Moqtada al-Sadr's Mehdi Army militia appeared in a show of force in a number of Baghdad neighbourhoods on Monday and forced shop owners to close, witnesses said.

Witnesses and Interior Ministry officials said Mehdi Army fighters spread through five districts in southern and western Bagdad, ordering shop owners to close.

In one neighbourhood, they closed the roads with flaming tyres. Witnesses said the militia had declared the start of a civil disobedience campaign.

There were no reports of any clashes with Iraqi or U.S. forces, but residents said the situation was very tense.

In at least one of the districts, southern Bayaa, witnesses said the Mehdi Army fighters were walking about unarmed, although guns could be seen in their vehicles.

An official in Sadr's office in one of the districts told Reuters that the Mehdi Army fighters were protesting against "U.S. raids and the arrests of innocent people". The militia has kept a low profile since Sadr called a ceasefire last August and extended it last month, a move U.S. commanders say has helped to sharply reduce sectarian violence between Iraq's majority Shi'ites and minority Sunni Muslims.

But gunbattles in Baghdad and the southern city of Kut last week have raised fears that it may be unravelling at a time when the U.S. military is withdrawing 20,000 troops.

Mehdi Army fighters, however, have complained that U.S. and Iraqi forces have exploited the truce to carry out indiscriminate arrests. U.S. commanders say they only target Mehdi Army cells that have ignored Sadr's ceasefire order.

"This does not mean the ceasefire is over. Such a decision is for Moqtada al-Sadr to take," Nassar al-Rubaie, the head of the Sadrist bloc in parliament, told Reuters, referring to the disobedience campaign.

On Sunday, the head of Sadr's office in northern Shula district gave the Iraqi government 24 hours to stop "all detention operations" and to release all Sadrists in detention.

"PROTESTS TO CONTINUE"

"U.S. forces mistakenly thought that the extension of the truce was a sign of weakness of the Mehdi Army. That is not true, they are still strong, but we are obeying the orders of Moqtada al-Sadr," Sheikh Mahmoud al-Sabihawi, an official in Sadr's office in Baghdad's Amil district, said on Monday.

"We will continue the protests until the arrests stop," he said.

Interior Ministry officials said Mehdi Army fighters had appeared on the streets of al-Ilaam, Shurta, Bayaa and Amil districts in southern Baghdad and in Washash in the western section of the capital in an apparently co-ordinated action.

"They forced shop owners to close, saying they were calling for civil disobedience. The situation is very tense. There are rumours that the situation is going to deteriorate," said Abu Adel, 55, a civil servant, and a resident of al-Ilaam district.

Adel Alwan, a resident of Amil district, said shops had also been closed in his area and Mehdi Army gunmen could be seen standing on street corners.

"I left the street and went back to my house and locked the door. I am anticipating the Americans will come and clash with them," he told Reuters. (Writing by Ross Colvin; Editing by Ibon Villelabeitia)

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