BAGHDAD — It was a tense confrontation between two forces supposed to be on the same side in Iraq.

First, heavily armed police, led by the interior minister, waded into a Shiite militia base south of Baghdad and arrested its deputy commander, accused of organizing attacks on Sunni mosques. They loaded the man, Ali Reda, into an armored SUV.

Then militia reinforcements descended, surrounded the police and demanded Reda be freed. Weapons were drawn. The minister, Mohammed al-Ghabban, the highest figure in Iraq’s police force, frantically called Baghdad from inside his SUV.

In the end, al-Ghabban surrendered his prisoner and left empty-handed, angry and humiliated.

The standoff in mid-January, described to The Associated Press by six different officials and militia leaders, was a stark example of the power that Shiite militias have accrued in Iraq and their boldness in wielding it.