(Reuters) - Iran has closed its borders with Iraq and urged its citizens to avoid travelling there, a senior official said on Tuesday, amid an eruption of violence after powerful Shi'ite cleric Moqtada al-Sadr said he would quit politics.

Heavy clashes in Baghdad killed at least 20 people on Monday, after Sadr's announcement prompted his loyalists to storm a government palace and fight with rival groups.

Millions of Iranians travel to the Iraqi city of Kerbala every year for the ritual of Arbaeen, which marks the end of a 40-day mourning period for the grandson of the Prophet Mohammad, Imam Hussein. Arbaeen falls on Sept. 16-17 this year.

"The border with Iraq has been closed. Due to safety concerns, it is necessary for Iranians to refrain from travelling to Iraq until further notice," state TV quoted Iran's deputy Interior Minister Majid Mirahmadi as saying.

State TV said Iran had halted all flights to Iraq "until further notice because of the ongoing unrest".

"We are trying to arrange an emergency flight to bring back Iranians from Iraq and Baghdad who are currently at the airport. We hope to evacuate them today," state TV cited a senior aviation authority as saying.