Thousands of mourners have joined the formal funeral processions for Iranian Major-General Qassem Soleimani and seven others who were killed in a US air strike in Iraq's capital, Baghdad.Dressed in black and raising the flags of the powerful paramilitary group Hashd al-Shaabi (Popular Mobilisation Forces or PMF), the large crowds first gathered near the Shia shrine of Khadhimiyya in Baghdad to pay their respects to the dead at the holy site.Soleimani, the head of Iran's elite Islamic Revolutionary Guard Corps' (IRGC) Quds Force and mastermind of its regional influence, was killed early on Friday near Baghdad's international airport in an air strike ordered by US President Donald Trump.The attack came just days after Hashd members and supporters attempted to storm the US embassy in Baghdad after the US launched air attacks against Kataib Hezbollah positions in Iraq and Syria. Al-Muhandis had been among the crowds of PMF members and supporters.The mourners, most of which include supporters of the PMF, an umbrella grouping of mostly Iran-backed armed groups, will then march through the Green Zone where government offices and foreign embassies, including the US embassy, are based. Mohannad Hussein, a media representative of the PMF, which organised the funeral processions, told Al Jazeera that the crowds will end their march at Hurriya Square in central Baghdad for members of the public to pay their final respects, before the bodies are taken to the holy Shia city of Karbala where funeral prayers will be held later on Saturday.Karbala is the base of Iraq's top Shia leader Grand Ayatollah Ali al-Sistani who condemned the US attack and called on all parties to practice restraint in a statement from his office for the Friday sermon.Hussein said the bodies will then be taken to the holy Shia city of Najaf where Muhandis will be buried later on Saturday along with other Iraqis killed in the attack. The body of Soleimani will be flown to Tehran for funeral processions on Sunday.Iran is observing three days of national mourning in honour of Soleimani who is widely believed to have been the second-most powerful figure in Iran. Supreme leader Ayatollah Khamenei promised to exact "harsh revenge" for the targeted killing."This is a very sad day for all of us. But each fighter [in the PMF] considers himself a martyr and so we are ready to give up our lives like our leaders did," said Hussein."The US has opened a new chapter in its relations with Iraq and made clear that it is the real enemy," said Hussein."As part of the military, the Hashd al-Shaabi will do what the government orders us to do. We are ready to fight," he added. The Iran-backed PMF was integrated into Iraq's armed forces last year, but critics say that some factions continue to operate independently of Baghdad. Iraq is poised for a tense period ahead, according to analysts, as top Shia leaders - many of whom are expected to attend the processions - warned of repercussions following the attack. Iraq's prominent Shia leader Muqtada al-Sadr called on his militias (Army of Imam Mahdi) and "other national and disciplined" armed groups to be prepared to protect Iraq, adding that the killing of Soleimani will not weaken Iraq's resolve.Qays al-Khazali, head of Asaib Ahl al-Haq armed faction, which is part of the PMF, said "all fighters should be on high alert for upcoming battle and great victory"."An end to Israel-US presence in the region will result from the assassination of Soleimani and Muhandis," al-Khazali said in a statement published by Iraqi media. "This is a dangerous time for Iraq as it moves into a period of greater instability and uncertainty," Renad Mansour, head of the Iraq Initiative at Chatham House told Al Jazeera. Mansour said Iraq would witness changes in the coming days. "The Iraqi parliament will most likely ask US forces to leave Iraq, while the US may become antagonist vis-a-vis Baghdad," explained Mansour.Iraq's parliament is due to sit on Sunday to discuss the US air strike, after Prime Minister Adel Abdul Mahdi called on lawmakers to hold an emergency session and address the killings of Soleimani and Muhandis at Baghdad airport, which he called a violation of sovereignty.