Several thousand supporters of one of the most powerful pro-Iranian armed groups in Iraq marched through Baghdad on Saturday, trampling on giant US flags after Washington slapped sanctions on their leader.

The international community has condemned the role of pro-Iranian factions in a crackdown on two months of protests in which nearly 460 people have been killed and 25,000 wounded.

Last week Washington imposed sanctions on three Iraqis including Qais al-Khazali -- who heads Asaib Ahl al-Haq, part of the Hashed al-Shaabi security force tied to the Iraqi state -- and his brother.

The US Treasury accused his group of "widespread forced disappearances, abductions, killings, and torture".

The measures block financial transactions with and travel to the US for people who commit human rights abuses or corruption.

Khazali has laughed off the sanctions as an "honour".

His supporters blocked major roads in the capital on Saturday, staging mock hangings using effigies of US President Donald Trump, Israeli Prime Minister Benjamin Netanyahu and Mohammed bin Salman, crown prince of Iran's arch-rival Saudi Arabia.

"We denounce the unjust decision" to punish Khazali, said Mahmoud al-Rubaye, a member of Asaib Ahl al-Haq's political wing.