Iraqi troops have surrounded an office block used by supporters of Muqtada al-Sadr, leader of the Shia al-Mahdi Army, in the southern city of Basra.
Al-Sadr's supporters criticised the move on Friday as a "provocation", but the government said its operation was aimed only at recovering offices unfairly occupied by political groups.
Sheikh Harith al-Athari, the head of the Basra office, said: "The police and the army have laid siege to Sadr's office in Basra.
"They have also stopped people from attending Friday prayers. The forces, backed by armoured vehicles, have asked us to leave the building."
General Abdel Karim Khalaf, an interior ministry spokesman, said the operation had been approved by Nuri al-Maliki, the prime minister.
He said that al-Maliki had ordered government troops to take possession of all government buildings in Basra within 48 hours.
Stand off
Journalists in Basra reported that al-Sadr supporters had refused to leave.
Salah al-Obeidi, a Sadr spokesman from Najaf, in central Iraq, called the move "a provocation that will induce people to resort to force".
"The objective of the security forces is to further aggravate tensions and it shows the government's claims that it is not targeting Sadr supporters to be false," he said.
The al-Sadr office in Basra is in an old Olympic committee building.
Earlier crackdown
The incident comes only weeks after clashes between Iraqi forces and the al-Mahdi Army, when al-Maliki launched a crackdown on Shia fighters in the port city.
The clashes quickly spread to other Shia regions of Iraq, killing and wounding hundreds of people.
The US military confirmed that, in a separate incident earlier this week, a unit of Iraqi troops abandoned their positions in Baghdad's Sadr City district, a poor Shia suburb of Baghdad where support for al-Sadr is strong, during a battle with the al-Mahdi Army.
The battle in Sadr City is seen by many as a key test for Iraq's army after the earlier botched Basra crackdown.