Angry relatives of missing Iraqi soldiers stormed the parliament building in Baghdad on Tuesday, attacked MPs and began a sit-in in its main chamber, an official said.
Anti-riot police were trying to force out the hundreds of protesters, who were demanding to know the fate of relatives who surrendered to jihadists in June, said the official, who present at parliament.
They also called for commanders to be held accountable, he added.
The demonstrators assaulted some MPs as well parliamentary staff, and smashed chairs in the cafeteria.
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Some 1,700 soldiers surrendered to the jihadist Islamic State, also referred to as ISIS group in June as its fighters seized second city Mosul and swept south towards Baghdad.
IS subsequently released photographs of dozens of men in civilian clothes apparently being executed by firing squad in a desert area, and said that it had killed hundreds in total.
Parliament was supposed to discuss the issue on Tuesday, but the building was stormed before the start of the session, which has now been postponed until Wednesday.
Security forces were swept aside by the initial IS-led onslaught, but are now performing better, retaking areas northeast of Baghdad with the backing of Shiite militiamen and Kurdish fighters.