By Kareem Raheem and Stephen Kalin
BAGHDAD (Reuters) - Iraqi security forces opened fire on protesters who stormed into Baghdad's heavily fortified Green Zone on Friday and entered the cabinet building, drawing calls for revolt from a powerful Shi'ite Muslim cleric.
Dozens of demonstrators were injured by tear gas and live fire, witnesses said. Some security personnel were stabbed, according to a military statement. Authorities could not immediately verify reports that several civilians had been killed.
The thousands of protesters included supporters of cleric Moqtada al-Sadr and people from other groups upset with the government's failure to approve anti-corruption reforms and provide security.
The government briefly imposed a curfew on Baghdad and authorities later said order had returned after what they called rioting at the Green Zone, which houses parliament, government buildings and many foreign embassies.
"Infiltrators exploited our forces' preoccupation with preparations for the Falluja battle to penetrate state institutions and cause chaos," the military said, referring to a city 50 km (30 miles) west of Baghdad controlled by Islamic State for more than two years.
Protesters occupied the cabinet building for several hours. Some held Iraqi flags and flashed peace signs near the insignia of the prime minister's press office and inside a meeting room.
The protesters eventually withdrew to Tahrir Square, but witnesses said security forces and unidentified gunmen opened fire there as well.
A military statement said riot police were "dealing with anyone trying to damage state institutions in accordance with the law".
Prime Minister Haider al-Abadi condemned the Green Zone breach and warned against chaos and strife in a late-night televised speech, saying: "The law must take its course with every transgressor."
Sadr expressed support for what he called a "peaceful spontaneous revolt" and condemned the government for "killing its children in cold blood".
His supporters, protesting parliament's failure to approve a non-political cabinet, also breached the Green Zone on April 30, storming the assembly complex and attacking officials before holding a 24-hour sit-in at a nearby square.
Parliament has not convened since then, crippling government as it grapples with an economic crisis brought on by low oil prices and an Islamist insurgency that constitutes the biggest security threat to the OPEC oil producer since a U.S.-led invasion toppled Saddam Hussein in 2003.
The protesters on Friday added to their grievances the authorities' failure to maintain security following a wave of bombings claimed by Islamic State this month in Baghdad which killed more than 150 people.
Sadr did not explicitly call for Friday's demonstration, where protesters chanted: "Oh army, the country is hurt! Don't side with the corrupt!"
Iraq's political crisis goes back to plans announced by Prime Minister Abadi in February to replace politically affiliated ministers with independent technocrats. Despite backing from the Shi'ite religious establishment, the proposal threatens to uproot a system of political patronage that makes for a public administration rife with corruption and has faced stiff resistance.
Abadi has warned the impasse was undermining Baghdad's security and could hamper Iraq's fight against Islamic State, which continues to control territory in the north and west.
Sadr, the heir of a revered clerical dynasty, says he backs the premier's plan and has accused other political groups of blocking the reforms to protect vested interests.
(Additional reporting by Saif Hameed and Maher Chmaytelli; Writing by Stephen Kalin; Editing by Ralph Boulton and James Dalgleish)