By Aws Qusay

BAGHDAD, May 24 (Reuters) - Supporters of Shi'ite cleric Moqtada al-Sadr warned the Iraqi government on Saturday that it was jeopardising a fragile truce, accusing security forces of attacking worshippers loyal to him in Baghdad and Basra.

Iraqi security forces fired shots to disperse worshippers in the southern oil port city of Basra during Muslim prayers on Friday and seized hundreds of Sadr supporters in southwestern Baghdad at about the same time.

Officials in Sadr's political movement, one of the biggest blocs in parliament, said on Saturday that police had targeted a mosque in Baghdad's Amil district, arresting 400 worshippers inside and outside the building during Friday prayers. The mosque is also Sadr's office in the area.

Sadrists in Basra said one person was killed and five wounded when Iraqi troops opened fire to prevent worshippers from gathering in a square.

Police said the soldiers had fired shots into the air to break up an illegal gathering and that six had been wounded.

"We consider this a new page in the targeting of Sadrists by the Iraqi government and the U.S. forces," Salah al-Ubaidi, spokesman for Sadr, said in the holy Shi'ite city of Najaf.

"This aggression on our Friday prayers is a new escalation which could have grave consequences for the future."

Sadrists said the government was violating recently agreed peace deals to end weeks of fighting between Sadr's Mehdi Army militia and U.S. and Iraqi forces in which hundreds have died.

The truces negotiated in Basra and Baghdad have largely held and are partly credited by U.S. forces for near record-low levels of violence countrywide in the past two weeks.

They allowed some 10,000 Iraqi troops backed by tanks to enter Sadr City, Sadr's main stronghold in Baghdad, unopposed this week, to stamp the government's authority over an area largely outside its control since coming to power in 2006.

"BREACH"

Sadrists, former allies of Prime Minister Nuri al-Maliki who pulled out of his government last year, held a news conference on Saturday to protest against the security force action and also met Defence Minister Abdel Qader Jassim in Baghdad.

A Defence Ministry spokesman and Sadr officials said the minister had promised to ensure troops respected Friday prayers.

"We have seen a serious breach we didn't witness even under the Baathist dictator," Sadrist lawmaker Hassan al-Rubaie told the news conference, referring to Saddam Hussein, whose Sunni Arab-dominated government suppressed Shi'ites for decades.

An Interior Ministry official defended Friday's raid in Baghdad, saying it had targeted Mehdi Army militiamen and that machineguns and rocket-propelled grenades had been seized.

He said hundreds were arrested but had no exact figure.

One resident who witnessed the raid told Reuters police had detained scores of worshippers inside the mosque and then made what he called random arrests at a market nearby.

A U.S. military spokesman, Lieutenant-Colonel Steven Stover, confirmed U.S. forces had been involved in the operation, which he said also seized four roadside bombs known as explosively formed penetrators that are powerful enough to destroy a tank. (Writing by Tim Cocks; Editing by Elizabeth Piper)

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