Up to a million followers of Iraqi cleric Moqtada Al-Sadr took to the streets on Monday in a massive show of force before an Arab League summit, which Iraq's long-oppressed Shia majority view as their debut on the regional stage.
The protest in the southern city of Basra marked the anniversary of the start of the US invasion in 2003. Slogans were mainly directed at the government of Prime Minister Nuri Al-Maliki for failing to improve the lives of Iraq's poor.
But the main context appears to be the 27-29 March Arab League summit, the first in Iraq in more than 20 years and the first ever hosted by a mainstream Shiite Arab ruler.
Basra police estimated the size of the crowd at between 700,000 and 1 million. A Reuters correspondent on the scene said it was clear at least several hundred thousand were there.
Sadr's followers frequently hold demonstrations denouncing Sunni rulers of other Arab states, especially Bahrain and its ally, Saudi Arabia, for cracking down on Shiite protesters last year. The last demonstration on 9 March saw Saudi flags burned.
Monday's demonstration, by contrast, seemed to show a deliberate effort to avoid antagonising Arab neighbours by channelling protesters' anger into domestic grievances.
There is no shortage of such outrage in a country where oil wealth has yet to alleviate dire poverty. Nine years after the invasion that toppled Saddam Hussein - a Sunni who oppressed Shia - most Iraqis have electricity only a few hours a day.
Men and women travelled from across Iraq to protest bearing Iraqi flags and portraits of Sadr. They carried black caskets labelled "Electricity," "Education" and "Democracy."
"We came to call for the removal of injustice against Iraqis ... There are no jobs. We are living in bad conditions without services," said Latiaf Kadhim, who came hundreds of miles from Kerbala to participate in the demonstration in Basra.
Muttashar Saeed, who had travelled from Baghdad's Sadr City neighbourhood, said: "Lawmakers are looking out for themselves while the state ignores the poor. We want the attention of officials who are busy with their own affairs in their comfortable chairs and armoured vehicles."
Although the rhetoric has an internal target, such a massive show of Shiite power highlights the awkwardness for some Sunni Arab neighbours in embracing Baghdad, which after Saddam's fall became the Arab world's only mainstream Shiite-led state.
All the leaders at next week's summit will be Sunni Muslims except Maliki. Syria's Bashar Al-Assad, a member of the small Alawite minority sect, is not invited because the Arab League suspended the membership of Damascus.
Sadr, who led uprisings against the US presence before American troops withdrew last year but is now a key member of Maliki's ruling coalition, has banned protests during the summit to show "hospitality" to the guests.
Holding Monday's protest in Basra rather than Baghdad helps Maliki's assertion that the capital is safe for the summit.
Authorities in Baghdad took no chances on Monday, imposing a security operation with checkpoints conducting thorough searches that snarled morning rush-hour traffic for hours.
Most of the world's Muslims are Sunnis, but Shia are the majority in Iran, Iraq, Bahrain and in pockets throughout the Middle East, including parts of Saudi Arabia and Lebanon.
The Middle East's Sunni-Shia divide, which led to sectarian civil war that killed tens of thousands of Iraqis in 2006-07, has intensified in other parts of the region, notably Bahrain, where a Sunni royal family rules a Shia majority, and Syria.
Iraq has been more reluctant than other Arab states to support the mainly Sunni opposition groups fighting Syria's Assad, and has been one of the few Arab states to criticise the crackdown on Shia protests in Bahrain.