The rights group said there's a pattern of attacks by Sunni extremist groups against members of the Shiite community in Iraq during the holy month of Muharram, which began Nov. 5. Ashura, the holiest day of the season, was Thursday.
"The government should do its utmost to enable Shiites to participate in holy month processions without fearing attack, but it needs to focus on thwarting attackers, and not besiege entire communities," Eric Goldstein, deputy Middle East director at Human Rights Watch, said in a statement.
The rights group said the security crackdown during the holy season was particularly harsh. It said special security teams raided homes and arrested people in droves ahead of Ashura festivities.
"Harassing entire neighborhoods exacerbates, not mitigates, the horrors Iraqis face on a daily basis," he said.
More than 40 Shiite pilgrims were killed Thursday in twin suicide bombings north of Baghdad as they gathered to celebrate Ashura, the rights group said.