Supporters of Shia Muslim cleric Moqtada al-Sadr have stormed Iraq's presidential office and clashed with security forces.

The protesters, whose number was estimated to be in the range of hundreds to thousands, broke into the office and other buildings in Baghdad on Monday.

Local media reported that at least several dozen people were injured.

In late July, al-Sadr's supporters stormed into parliament and skirmished with security forces, leaving more than 120 people injured. They went on to stage a sit-in outside the parliament building that lasted a month.

A group led by al-Sadr won the largest number of seats in last October's parliamentary election, but not enough to secure a majority government.

His bloc has been unable to pick a president and prime minister amid stalled negotiations to form a coalition government. More than 70 lawmakers from the group resigned in June, as they called for prompt elections.

The latest clash with security forces was apparently triggered by al-Sadr's announcement on Twitter that he would resign from politics. The move is seen as an attempt by al-Sadr to tilt things in his favor.

Iraq's security authorities have announced a nationwide curfew in a bid to quell the unrest.