The storming of the parliament building in Baghdad by protesters chanting the name of populist Shia cleric Muqtada al-Sadr is a sign that the political system built up since the US invasion in 2003 is disintegrating. The Iraqi security forces stood back and did nothing as the protesters burst into the Green Zone, graphically illustrating the weakness of Prime Minister Haider al-Abadi and of state institutions generally.

The eruption by Sadrist supporters into the heavily fortified Green Zone, the heart of government power in Iraq, came minutes after Mr Sadr ended a press conference in the Shia holy city of Najaf during which he condemned politicians for refusing to end the political stalemate. He has been demanding that Mr Abadi appoint a non-sectarian government of technocrats that would end corruption and other abuses. Since early in the year, he and his followers have been holding mass rallies but he has previously restrained them from invading the Green Zone, though it was always likely that the security forces there would not stop them.

It is unlikely that Mr Sadr wanted the invasion of parliament because it will lead to further weakening of the government rather than its reform. In the past, he has alternated between making moderate demands, but at the same time threatening that the anger of the people could not be contained. He himself, symbolically, entered the Green Zone a month ago, but instructed his followers not to follow him.

Sadrist protestors attacking Fadhila MP Ammar Tuma. Shia-Shia conflict so far. #pt pic.twitter.com/Rb0sRpTt8e — Hayder al-Khoei (@Hayder_alKhoei) April 30, 2016

Mr Sadr comes from a family of Shia clerics that owes its iconic and almost semi-divine status in Iraq to its long resistance to Saddam Hussein and the Baathist regime. He was 25 in 1999 when his father, Mohammed Sadiq al-Sadr, and his two brothers were assassinated by Saddam Hussein’s gunmen. They had led a populist religious movement drawing support from the poor as well as many tribes that had become visibly hostile to the regime in Baghdad.

Muqtada al-Sadr remained under house arrest in Najaf until 2003 when he emerged as the leader of a movement that opposed the US occupation and fought against it in 2004.

The Mahdi Army, that battled the US army in Najaf, later played a leading role in the sectarian war in Baghdad in 2006-7 in which tens of thousands died. Mr Sadr later disavowed many of his followers who carried out sectarian killings and retired to Iran to carry out religious studies. But on his return to Iraq, he continued to lead a political, powerful and well organised movement that elected an important group to the Iraqi parliament and held several ministerial posts.

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There were reports of protesters leaving the Green Zone, but they may not all go and what has happened once can happen again. Foreign embassies may pack up and go because they fear they will be targeted next time round for political reasons or simply as places to loot. The other danger is that there may be units in the Iraqi army such as the elite Golden Brigade – that might shoot down demonstrators. The other main Shia political factions are also capable of mobilising their own militias to defend their interests, which they see as being threatened by the Sadrists. This may lead to battles between the different armed groups.