Baghdad, March 7 (DPA) Iraqis headed to the polls amid heavy security Sunday morning to cast their votes in the second parliamentary election since the US overthrow of Saddam Hussein's regime in 2003.

Three people were killed by a car bomb Saturday in Najaf, the interior ministry said. More than 50 other people, including 37 Iranians, were injured in the attack near the Imam Ali Holy Shrine, the tomb of the Prophet Mohammed's cousin Ali.

Nearly 6,300 candidates are competing for 325 seats in parliament, making it the largest parliamentary election in Iraq's history.

The Iraqi Independent High Electoral Commission (IHEC) says it expects some 18.9 million Iraqis to cast their votes. Over 500,000 local and international observers are monitoring the election process.

The new parliament, elected for a four-year term, will be responsible for forming a new government as well as naming the country's next president and parliamentary leader.

Iraqi authorities imposed a curfew from Saturday night until 5 a.m. March 8, which restricts movement of vehicles and closes air, land, and sea ports.

Over half a million members of Iraqi security forces are working to maintain peace during the elections.

The weeks leading up to voting were fraught with sectarian and political violence, as well as a large number of attacks targeting Iraqi security forces.

Early voting, which took place Thursday for Iraqi soldiers, police, hospital patients and workers, and prisoners serving fewer than five years in prison, was marred by attacks which left 12 people dead and 26 injured.

The attacks targeted different polling stations in Baghdad, and one candidate's house in Mosul.

The violence came amid allegations from the head of Iraq's electoral commission that security forces in Mosul were urging voters to cast their ballots for Prime Minister Nuri al-Maliki.

Some among those eligible to vote Thursday found they could not do so because their names were not on the voter-registration lists.

The interior ministry said two of the people killed in Saturday's bombing were Iranians. Najaf is located about 160 km south of Baghdad, and attracts many Shiite pilgrims from Iran.

Iraqis living abroad began casting their votes March 5 and will continue to do so Sunday.

Around two million votes are also expected from Iraqis living in 16 countries - Jordan, Syria, Lebanon, Egypt, United Arab Emirates, Iran, Turkey, US, Germany, Britain, Holland, Sweden, Denmark, Canada, Austria and Australia.