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.

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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 Islamic State in Iraq and the Levant (ISIL) group.

Al Jazeera's Imran Khan, reporting from Baghdad, said Sadr was keen to emphasise 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 Muslims.

Both the Iraqi government and Shia religious authorities have called on Iraqis to volunteer to fight a Sunni rebellion that has taken over big chunks of the country, including the cities of Mosul and Tikrit.

Police officials told Al Jazeera that improvised explosive devices, or IED's, were detonated in four areas across Baghdad's Shia-populated neighbourhoods. At least nine people were killed and 15 injured.

On Saturday, rebels led by ISIL seized a border crossing with Syria near the town of Qaim about 320km from Baghdad, leaving about 30 Iraqi soldiers dead, the AP news agency reported.

AP also reported that the town of Rawah in Anbar province had been captured by ISIL fighters later the same day, citing the town's mayor. He added that local army and police forces had pulled out when the fighters took control.

Pressure on Maliki

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.

Al Jazeera's Imran Khan reports from Baghdad

Iraqi Prime Minister Nouri al-Maliki has faced pressure from inside and outside the country to form an inclusive government, to prevent Sunni Muslims 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, US 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.

Source: Al Jazeera and agencies