Baghdad ? Violence left nearly 50 people dead in two major southern cities Friday when members of a shadowy, messianic cult attacked police and fellow Shiite worshippers – a year after a similar plot was foiled during Shiite Islam’s most important holiday.

Iraqi authorities said at least 36 people were reported killed in Basra, Iraq’s second largest city, and at least 10 in Nasiriyah, where witnesses said U.S.-led coalition jet fighters and helicopter gunships targeted a police station seized by cult gunmen.

U.S. military spokesman Maj. Brad Leighton said an Iraqi request for air support in the area was approved, but he could not confirm whether airstrikes were carried out. Some clashes raged into the night, raising the possibility of more casualties.

The assaults were launched as hundreds of thousands of Shiites observed the Ashoura holiday by marching, singing and beating their chests to honor the martyrdom of their most beloved saint. Followers of the cult – the Soldiers of Heaven – seek to speed the return of another Shiite figure known as a “Hidden Imam,” who believers say will bring justice to the world.

A series of recent high-profile attacks is eroding the security gains of the previous six months, when violence dropped across much of the country. The main insurgent group, al-Qaida in Iraq, has carried out many of the attacks against fellow Sunnis who have turned against it. But insurgents also struck with deadly suicide blasts this week against Shiites observing Ashoura.

Friday’s clashes pointed to a third problem that shows no signs of easing: Shiites attacking fellow Shiites. The attacks were a reminder of persistent divisions within the Shiite community at a time when the Pentagon is claiming some success in calming armed opposition among the Sunnis.

Shiite factions have been engaged in a power struggle across southern Iraq. On Friday, two prominent Shiite leaders issued fresh challenges to the Shiite-led government in Baghdad.

Radical cleric Muqtada al-Sadr warned he may not extend a six-month cease-fire by his Mahdi Army militia, because security agencies are packed with “criminal gangs,” his spokesman said in reference to Shiites from rival groups. The group’s cease-fire, due to expire next month, has been a major factor in the reduction in violence.

And Abdul-Aziz al-Hakim, the head of parliament’s largest Shiite political bloc, accused Prime Minister Nouri al-Maliki’s government and legislators of allowing “personal whims” to delay national unity.