The Iraqi cabinet has decided not to authorise a protest camp by followers of cleric Moqtada Sadr which had been due to begin Friday in front of the fortified "Green Zone".

Tens of thousands of Sadr supporters have been preparing for a days-long sit-in in central Baghdad aimed at pressuring the government to implement deeper political reform.

"Staging a sit-in is not permitted by law, especially in the current security circumstances, notably the threat by terrorist groups and the potential for this gathering to be targeted," a cabinet statement said Wednesday.

"The security forces are busy with the fight against Daesh (Islamic State group) and it is not possible to guarantee the protection of this gathering at all times," it said.

The cabinet stressed that it "supports the demonstrations demanding government reforms" and has protected one-day protests by the Sadr movement in recent weeks.

The Najaf-based Sadr had urged his supporters on March 12 to prepare tents for a protest camp supposed to begin on Friday and last until the expiry 10 days later of an ultimatum he gave the government.

A month ago, the Shiite cleric gave Prime Minister Haider al-Abadi 45 days to present the names of technocrats for a new government.

Sadrists have held a series of massive rallies in central Baghdad, with thousands turning out last week for the latest protest to press their demands.

The presence a week earlier of armed Sadr supporters outside the Green Zone -- where Abadi's office, parliament and the US embassy are located -- sparked intense security concerns.

In the protest that he attended in Baghdad three week ago, Sadr threatened that his supporters could storm the Green Zone if their demands were not met.