Protesters stormed Iraq's Parliament in a dramatic culmination of months of demonstrations, casting uncertainty over the tenure of the country's leader and the foundations of the political system laid in place after the 2003 US-led invasion.

Security forces declared a state of emergency in the Iraqi capital after demonstrators climbed over blast walls and broke through cordons to enter Baghdad's fortified Green Zone, also home to ministries and the US Embassy.

Many were followers of Iraq's powerful Shiite cleric Moqtada al-Sadr, who has been urging his supporters onto the streets.

MPs fled the building in panic, with some berated and hit as they left. Others were trapped in the basement for hours, too afraid to face the crowds who complain that the country's political class is racked by corruption.

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It was a day of high drama for a country that is no stranger to revolution and that has seen all of its leaders overthrown from the time the state was established in 1921 until the US invasion toppled Saddam Hussein in 2003.

At issue now is the quota system introduced when the US-led coalition put together Iraq's first post-invasion government, which determines Iraq's political positions according to sect and ethnicity.

The turmoil threatened to unseat the already embattled Prime Minister, Haider al-Abadi, with whom the United States has partnered in the fight against Isis (Islamic State) militants in Iraq but whose efforts at reform have stumbled.

US officials have expressed concern that the unrest could have impact on the battlefield as Iraq also struggles with an extreme budget crisis caused by a plunge in oil prices.

"Today the people announced their revolution," said Sadr, who led a violent resistance against US troops during the Iraq War. "History will record the birth of a new Iraq, from the ashes of corruption and the corrupt."

Entering the Parliament building, which, like the rest of the Green Zone, has been off-limits to the public for the past 13 years, protesters reacted with jubilation.

To many, the area has become a symbol of corruption, the place where Iraq's political elite live walled off from the rest of the country.

They crammed the building's main hall, chanting and waving flags.

"I was thrilled to be in that room. It was like being in a place you only see on television," said 26-year-old Abdullah al-Zaidi. "When I entered I was looking at chairs, and I wanted to break them because the politicians are killing us and stealing from us from these chairs."

He said he didn't, as organisers urged protesters to remain peaceful. However, television footage showed that some others did.

As evening fell, Zaidi and thousands of other protesters had moved their sit-in to a parade ground near the Hands of Victory monument - an arch shaped from crossed swords and one of the capital's most famous landmarks - putting them less than a kilometre from the US Embassy compound.

The embassy denied that it had begun evacuating staff or had provided safe haven for Iraqi politicians. Organisers urged protesters not to attack embassies or other properties in the area.

Still, MPs, many of whom live in the Green Zone, went into hiding, and some left the country.

Shwan Dawoodi, a Kurdish MP, said they were "scared for their lives."

"What's happening in the streets is terrifying," he said by phone. He said he was attacked as he left Parliament.

Ammar Toma, a politician with the Shiite Fadhila Party, was also accosted as he tried to leave. "Hit him, hit him!" one protester shouted.

Five Kurdish MPs were rescued by Kurdish peshmerga forces after locking themselves in a room in the building for safety.

One video posted online showed protesters riding around on a military vehicle.

Many politicians said the day's events marked a turning point.

"This is an end to the political system put in place after 2003," said Dawoodi, speaking by phone after he fled the Parliament building. "A big part of the blame for this is on America, which left Iraq without solving this crisis it created."

He blamed the Prime Minister for putting MPs' lives at risk, after local television reports cited Abadi as saying he had ordered that protesters be allowed into the Green Zone. Abadi's office issued a statement denying that he had done so.

A soldier at one gate into the Green Zone recounted an exchange between security forces trying to hold back protesters near Parliament. "There were orders to stop them and to not allow them to enter, but the soldiers responded that they couldn't," he said.

Shia cleric Moqtada al-Sadr speaks in Najaf. Photo / AP
Shia cleric Moqtada al-Sadr speaks in Najaf. Photo / AP

At another gate, security forces pushed back demonstrators with tear gas as the sound of gunfire broke out, video from the scene showed.

"That's it for Abadi," said parliamentarian Hisham al-Suhail, who said dozens of MPs were exchanging messages saying the Prime Minister is not fit to run the country.

"What's happened has ruined everything we've built over the last 13 years. The Green Zone represents the whole government of Iraq."

The speaker of Parliament, Salim al-Jubouri, who a group of rebel parliamentarians attempted to oust in a vote last week, said the attack on the legislature was an "attack on the prestige of the state," with its members "servants" of the Iraqi people.

But protesters say that Iraqi politicians are corrupt and stealing the county's oil wealth.
Street protests began last northern summer, when tens of thousands demonstrated against corruption and a lack of services.

They were reinvigorated when Sadr put his weight behind them earlier this year, calling for Iraq's government to be replaced by technocrats.

Under immense pressure, Abadi has tried to reshuffle his cabinet and meet the demonstrators' demands.

But he has been hampered by a deeply divided Parliament, and sessions have descended into chaos as MPs have thrown water bottles and punches at one another.

Protesters outside have watched the school-yard antics with growing impatience.

Earlier in the day, not enough MPs had turned up to officially convene a session in which Abadi was due to present names for a cabinet reshuffle.

Sadr held a news conference from the southern city of Najaf.

"They are against reform. They hope to behead the will of the Iraqi people," he said of the country's politicians. "I'm with the people, no matter what they decide. I'm standing and waiting for a major uprising of the Iraqi people."

Shortly afterward, protesters pushed through the multiple security cordons around Parliament.

After the breach, security forces announced that they had closed all roads into the capital, but they did not go as far as to reinstate a curfew.

"We are in the highest degree of readiness," said Colonel Ibrahim al-Baydani, a spokesman for the military, noting that Baghdad was already on high alert because of a Shia pilgrimage. Isis attempted to bomb pilgrims on the route . Unable to get near the pilgrims, the suicide bomber detonated a truck bomb among crowds at a busy sheep market instead, an official said.

Abadi called for protesters to return to areas of the capital that have been designated for demonstrations. The security situation, he said, was "under control."

But young Iraqis used Facebook's "check in" feature to mark their locations at landmarks in the Green Zone, in what was for some of them their first visit to the heart of the capital.