Thousands of Shia Muslims are taking part in rallies across Iraq vowing to protect their religious sites in a show of power that had been called for by influential Shia leader Moqtada al-Sadr.
The largest rally took place in the northern Baghdad suburb of Sadr City, where hundreds of men dressed in combat fatigues and carrying assault rifles marched in military formation.
Sadr is believed to have command of more than 10,000 fighters, most of whom have volunteered to fight alongside Iraqi security forces against Sunni rebels led by the militant group the Islamic State in Iraq and the Levant (ISIL).
Sadr seems keen to emphasize that his fighters would only serve as a defensive force to protect Baghdad, but there are fears of a reestablishment of the Mahdi Army, which was disbanded in 2008.
The reemergence of the Mahdi Army, which was accused of involvement in Iraq's sectarian conflict between 2006 and 2008, would heighten fears of a broader war between Sunni and Shia Iraqis.
Both the Iraqi government and Shia religious authorities have called on Iraqis to volunteer to fight a Sunni insurgency that has taken over big chunks of the country, including the cities of Mosul and Tikrit.
Many Sunni Muslims in western Iraq have supported the rebellion led by the fiercely anti-Shia ISIL, because of perceived anti-Sunni policies by Iraq's Shia-dominated government.
Iraqi Prime Minister Nouri al-Maliki is facing pressure from inside and outside the country to form an inclusive government, to prevent Sunni Iraqis from joining forces with ISIL.
In a thinly veiled rebuke of Maliki, the country's highest Shia authority, Grand Ayatollah Ali al-Sistani, called for a "broad" government that would "avoid past mistakes."
Such criticism from Iraq's most revered religious leader could force Maliki to step aside.
On Thursday, President Barack Obama all but called on Maliki to resign, saying only a leader with an "inclusive agenda" could end the crisis.
Maliki, whose State of Law electoral slate won most seats in April's election, has yet to form a majority coalition in the new 328-seat legislature, which must meet by June 30.
Meanwhile the Sunni insurgency has captured Iraq's biggest oil refinery after overnight clashes with Iraqi security forces, according to local sources, but a military spokesman denied it.
A journalist in Saladin province told Al Jazeera that Sunni rebels, led by the Islamic State in Iraq and the Levant (ISIL), had seized the refinery at Baiji, about 27 miles north of Tikrit, on Saturday.
An unknown number of soldiers had been taken prisoner by the rebels after the fall of the facility, the journalist said.
The oil ministry did not comment on the report and referred all inquiries to the defense ministry.
Rebels had withdrawn from the refinery on Friday after heavy clashes and retreated to Baiji's main town, which they already control.
State TV reported that Iraqi forces, backed by combat aircraft, had repelled four attacks on the refinery by ISIL fighters.
Iraqi security forces have largely halted the initial rapid advance by ISIL-led fighters, but the rebels continue to make gains.
On Friday, Sunni fighters reportedly captured the Qaim border crossing with Syria, about 199 miles west of Baghdad, after a day of clashes that killed about 30 Iraqi soldiers. Sameer al-Shwiali, media adviser to the commander of Iraq's anti-terrorist squad, told Reuters, however, that the Iraqi army was still in control of Qaim.
Al Jazeera and wire services, with reporting by Imran Khan in Baghdad.