Thousands of supporters of prominent Iraqi cleric Moqtada Sadr defied a government ban Friday to launch sit-ins at the main gates of Baghdad's Green Zone aimed at pushing for reforms.

Many of the demonstrators carried Iraqi flags as they muscled past tight security and set up tents to begin what they said was an open-ended protest.

"The sit-ins have started in front of the Green Zone gates as a message to the corrupt people who live there," Ibrahim al-Jaberi, a local official from Sadr's movement, told AFP.

The Najaf-based Sadr has called on his supporters to remain in front of the fortified "Green Zone" until his demands are met.

The young Shiite cleric has demanded Prime Minister Haider al-Abadi reshuffle the cabinet to bring in technocrats and threatened a no-confidence vote in parliament if he failed to do so soon.

"The sit-in is open-ended," said Jaberi, as Sadr followers started setting up camp on the streets and under trees.

The vast restricted area in the heart of the city is home to most key institutions, including the prime minister's office, parliament and the US embassy, which is the world's largest.

Demonstrators chanting slogans such as "Yes, yes to reforms" moved to crossroads around the Green Zone and started setting up tents, rolling out mats and pulling blankets out of bags.

"We and all the people demand improvement in the country, a solution to corruption and the sacking of all those who stole our money," said Abu Hassan, 65, sitting by a tent with his three brothers and two of his sons.

His walking stick by his side, the man from Sadr City, a huge Shiite neighbourhood in northern Baghdad where Moqtada Sadr is very popular, said: "He told us to hold a sit-in, so we will stay here years if that's what it takes."

Sadr promptly issued a statement claiming victory in his tussle with the authorities and thanking God "for letting the will of the people triumph".

- Peaceful protest -

The move was in defiance of a cabinet decision denying the rally the necessary permits and an interior ministry warning not to provoke the security services.

Sadr had issued a statement on Thursday saying his movement would ignore the ban but also calling on his supporters to refrain from violence.

Sadr heads a militia called Saraya al-Salam (Peace Brigades) that had caused strong concern when it deployed armed men during a previous protest in Baghdad.

Amid fears the stand-off could escalate, Iraqi security forces have locked down Baghdad, the Arab world's second most populous capital with an estimated eight million residents.

"All entrances to Baghdad have been blocked and some main streets and bridges are also closed, especially those leading to the Green Zone," a police colonel said.

A group of demonstrators clipped the barbed wire on one of the bridges over the Tigris river to reach an entrance to the sprawling Green Zone but no violence ensued.

In his statement, Sadr praised the behaviour of the riot police and army forces deployed en masse to protect the Green Zone.

"The cooperation of the security forces exceeded all the expectations of some corrupt people who had bet against it," he said.

The 42-year-old scion of an influential clerical family rose to prominence when he launched a Shiite rebellion against US troops following their 2003 invasion of Iraq.

He had lost some of his political influence in recent years but has brought himself back into relevance with a series of rallies against corruption.

Senior politicians from his own Ahrar bloc are perceived as some of the most corrupt in Iraq but the mercurial leader has recently distanced himself from them.

He is seen as a nationalist with fewer ties to neighbouring Iran than many of the country's other leading Shiite politicians.