Iraq's top Shiite imam urged the government Friday to press ahead with reforms, but cautioned that the anti-corruption campaign must not come at the expense of the fight against ISIS and should remain within the boundaries of the law. In a sermon delivered by a deputy, Grand Ayatollah Ali al-Sistani said the government had no choice but to continue the reforms and must swiftly go after the "big thieves of public funds." The Iranian-born imam also cautioned against the reform campaign distracting authorities from the fight against ISIS, describing it as the "most honorable and righteous battle because it is fought in defense of our existence and future." "The battle for reform must not distract us from that fight," he said in comments delivered at the holy city of Karbala, south of Baghdad, by imam and Sistani representative Ahmad al-Safi. His warning against distracting authorities from the anti-ISIS fight appeared inspired by the struggle of the military and its allies to regain territory captured by the extremist group in the vast Anbar province west of Baghdad and its recent advances on the southern parts of the oil refinery town of Beiji, liberated from ISIS control last November. Also, Mosul, Iraq's second-largest city, has been under ISIS control since it was captured in June last year in a blitz that gave the group control of about a third of Iraq and neighboring Syria. Shiite Prime Minister Haider al-Abadi has won parliamentary support to trim his government and vowed to fight corruption, amid rising popular discontent over poor services. He also declared his intention to seek a popular mandate to amend the constitution, adopted in 2005. "It is incomplete," he said. Later Friday, thousands rallied in Baghdad and a string of other cities to press demands for reforms, better services and an end to corruption. Baghdad's rally attracted up to 10,000, many waving Iraq's red, white and black flags and chanting the national anthem. There were much smaller rallies in the southern city of Basra and the holy Shiite cities of Najaf and Karbala south of Baghdad. In Baghdad, hundreds of army and police cordoned off the rally, sealing off roads leading to the rally in the central Tahrir Square as army helicopters hovered above. "The battle for reforms we are fighting these days is also a vital one that will determine our and the nation's future. We, people and government, have no choice but to win that battle," Sistani said.