By Waleed Ibrahim

BAGHDAD, June 17 (Reuters) - Iraqi authorities have arrested several people who may be connected to the murder of a Sunni Muslim politician, officials said on Wednesday, a killing that rattled efforts to keep the peace between Sunnis and Shi'ites.

Brigadier General Numan Dakhil, commander of a police rapid reaction force in Baghdad, said a suspected senior member of the al Qaeda-affiliated Islamic State of Iraq was captured on Wednesday in the capital.

"His name is Ahmed Abed Awaied, and he is the mastermind of the parliament member's assassination," Dakhil said.

Harith al-Ubaidi, head of the biggest Sunni bloc in parliament and a leading human rights advocate, was shot dead at a mosque in west Baghdad after Friday prayers. [ID:nLC462379]

Other officials were more cautious in their assessments.

"In the last couple of days we have arrested some people involved in the case. They are now being subjected to a broad interrogation. We don't want to be rushed into saying they are the murderers," said Interior Ministry spokesman Major General Abdul-Karim Khalaf.

"But there are strong indications and the facts we have gathered so far have given us important clues. Our aim is to get those who ordered this and who are behind this crime."

Ubaidi had been seen as an important moderate able to broker peace among his bloc's groups and also with Shi'ites, Kurds and others who have vied bloodily for power since the 2003 U.S.-led invasion.

The government blamed the murder on al Qaeda, which views Shi'ite Muslims as heretics.

Al Qaeda and Sunni hardliners accuse members of Ubaidi's Accordance Front of being traitors for taking part in the political process with a Shi'ite-led government.

But Ubaidi was also a leading rights defender, in particular of Iraqi prisoners, and some fellow politicians have tried to point the finger at other potential culprits, such as Interior Ministry officials.

The assassination raised concerns it could undermine efforts for reconciliation in Iraq, which holds a parliamentary election next January.

The vote will be a test of whether Iraq's feuding factions can live in peace after six years of sectarian bloodshed, and as U.S. forces draw back in preparation for a full withdrawal by the end of 2011. (Additional reporting by Khalid al-Ansary; Editing by Michael Christie and Jon Boyle)

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