There has been a bloody start to Iraq's second parliamentary election since the 2003 invasion, with at least 24 people being killed in attacks.

At least two buildings have been destroyed and dozens of mortars fired across Baghdad and elsewhere.

The border with Iran has been closed, thousands of troops deployed, and vehicle movement has been banned.

PM Nouri Maliki called on voters to turn out in large numbers, saying that participation would boost democracy.

The election is taking place against a backdrop of much-reduced violence, with casualty figures among civilians, Iraqi forces and US troops significantly lower than in recent years.

But hundreds of people are still being killed each month, corruption is high and the provision of basic services such as electricity is still sporadic.

In one attack, 12 people were killed and eight injured when an explosion destroyed a residential building in northern Baghdad, officials said, shortly after another blast in the city killed five others.

Seven died in other attacks across the country, but no polling stations are reported to have been hit.

Sporadic mortar fire could be heard across the capital after polls opened at 0400 GMT, two bomb blasts were reported near a polling station in Fallujah, and there were also reports of mortar rounds being fired in Salahuddin province.

Islamic militants had pledged to disrupt the voting process with attacks - a group affiliated to al-Qaeda distributed leaflets in Baghdad warning people not to go to the polls.

A vast operation, involving more than half-a-million members of Iraq's combined security forces, has been put in place to try to prevent attackers from disrupting the election.

Most of the mortars were fired from Baghdad's predominantly Sunni districts, said the city's security spokesman, Maj Gen Qassim al-Moussawi.

"We are in a state of combat," he said. "We are operating in a battlefield and our warriors are expecting the worst."

But despite the hail of attacks, he said a car ban aimed at stopping car bombs had been lifted after four hours of voting, Reuters reported. Curbs on buses and lorries remained in force.

'Important choice'

Some 19 million Iraqis are eligible to elect 325 members of parliament, and polls will close at 1400 GMT unless voting hours are extended.

Mr Maliki told the BBC that the violence should not deter voters from turning out.

"What happened will push voters to take part in the election," he said.

"Most of those attacks are designed to psychologically terrorise the voters and prevent them from going to the polls.

"But it is well-known that Iraqis when they are challenged by terror, challenge it back."

In some neighbourhoods, mosque loudspeakers are exhorting people to go out and vote, and voters seem to be heeding the calls.

In Azamiyah (northern Baghdad), Walid Abid, 40, cast his vote to the crumple of mortars exploding not far away.

"I am not scared and I am not going to stay put at home," said the father-of-two.

"Until when? We need to change things. If I stay home and not come to vote, Azamiyah will get worse," AP quoted him as saying.

The previous election, in 2005, saw Mr Maliki become prime minister with Shia Muslim parties dominating the legislature.

President Jalal Talabani, seeking another term, was among the first to vote on Sunday in the Kurdish city of Sulamaniyah, and said the election marked both a step, and a test, on Iraq's march to democracy.

In a rare public appearance, radical Shia cleric Muqtada al-Sadr, speaking in neighbouring Iran, urged Iraqis to vote and to reject violence.

Iraq's last elections were in February 2009, when voters chose local representatives.

Expats crucial?

Sunday's elections are being seen as a crucial test for Iraq's national reconciliation process ahead of a planned US military withdrawal in stages.

US President Barack Obama plans to withdraw combat forces by the middle of this year and all US troops are expected to be out of Iraq by the end of 2011.

Correspondents say Prime Minister Maliki looks likely to retain power at the head of his Shia-led coalition.

The key will be whether Mr Maliki can bring Iraq's embittered Sunni minority into his government and make them feel they have a stake in Iraq's political future again.

Expatriate votes cast in Jordan and Syria could play a deciding role in a tight election race, counting for around 10 seats.

There was a reportedly high turnout, with estimates suggesting 800,000 people cast ballots.

Source BBC