Clashes between the militia and Iraqi security forces, mainly fuelled by tribal divisions, killed at least 25 people last week in Amara, which was handed over by British troops to Iraqi security forces two months ago.
Mohammed al-Askari, a defence ministry spokesman, said: "We have imposed the curfew due to the security situation there."
Al-Askari said that the defence ministry was redeploying the army and the curfew, which will remain in force until further notice, would allow soldiers to control Amara without problems.
In Baghdad, Nuri al-Maliki, Iraq's prime minister, issued a warning to the militias to "refrain from any armed action that violates security".
"The Iraqi government calls on the people of Meysan in particular to be wary of attempts to make our united people fight each other... Let all know that the government will deal firmly with those that disrespect the law," he said.