“I hereby announce my final withdrawal,” Sadr said in a statement posted on Twitter.
He criticized fellow Shi’ite political leaders for failing to heed his calls for reform. He did not elaborate on the closure of his offices, but said that some of his cultural and religious institutions would remain open.
Sadr withdrew his lawmakers from parliament in June after he failed to form a government of his choosing. A political impasse between him and Shi’ite rivals close to Iran has given Iraq its longest run without a government.
Supporters of Sadr have since the end of July occupied parliament and protested near government buildings, halting the process to choose a new president and prime minister.
Monday’s announcement raised fears that Sadr’s supporters might escalate their protests, fuelling a new phase of instability in Iraq.