Officials inspect the scene after a car bomb exploded near a bus for pilgrims in the Shia holy city of Najaf in Iraq on Saturday. (Alaa al-Marjani/Associated Press)

Iraqis went to the polls Sunday under tight security designed to thwart insurgent attacks in an election testing the ability of the country's democracy to move forward amid uncertainty over a looming U.S. troop withdrawal.

Almost 20 million voters are eligible to turn out for the election, only the second vote for a full term of parliament since the 2003 U.S.-led invasion seven years ago this month. About 6,200 candidates are competing for 325 seats in the new parliament.

Insurgents have vowed to disrupt the elections with violence. Baghdad police said four people were killed when two mortar shells landed in a neighbourhood in northeastern Baghdad in the early morning. At least 10 people were reported to have been wounded.

And Reuters news agency is reporting that at least 12 people were killed and eight wounded after an explosion destroyed a second building in Baghdad.

Insurgents also launched three mortars toward the Green Zone — a heavily fortified area home to the U.S. Embassy and the prime minister's office — almost perfectly timed to coincide with polls opening at 7 a.m. across the country, Baghdad police said.

But security was very tight across the capital, where only select authorized vehicles were allowed on the streets and voters headed to the ballot box on foot.

The borders have been sealed, the airport closed and hundreds of thousands of Iraqi military and police were on the streets. Still, at least three explosions could be heard in the early-morning hours, although there was no information about whether anyone was hurt.

At one polling place in Baghdad's Karradah neighbourhood, draconian security measures were in place with the school ringed by barbed wire, armed guards around the perimeter and police using metal detectors to scan voters.

The election has been viewed by many as a crossroads at which Iraq will decide whether to adhere to the sectarian politics — Shias aligning with Shias, Sunnis with Sunnis and Kurds with Kurds — that have defined its short democratic history.

They could also try to move away from the sectarian tensions that almost destroyed this Shia majority country that was held down under Saddam Hussein's Sunni-minority rule.

Al-Maliki faces Shia groups

Prime Minister Nouri al-Maliki is fighting for his political future against a coalition led by mainly Shia religious groups — the Supreme Islamic Iraqi Council and a party headed by anti-American cleric Muqtada al-Sadr.

He also faces a challenge from secular alliance led by former a secular Shia, Prime Minister Ayad Allawi, who has teamed up with a number of Sunnis in a bid to claim the government.

President Jalal Talabani was among the first to vote Sunday morning in the Kurdish city of Sulamaniyah. Talabani's party, the Patriotic Union of Kurdistan, is in a tight race with an upstart political party called Change that is challenging the two Kurdish parties that have dominated Iraqi politics for years.

Iraq's second nationwide election for a full parliamentary term comes at a vastly different time than the first in December 2005.

The U.S., which has lost more than 4,300 troops in the nearly seven-year conflict, now has fewer than 100,000 troops in the country and their presence on the streets has all but vanished. The monthly American death toll has plummeted.

Overall violence is down dramatically, although attacks continue and insurgents have threatened voters.

A car bomb targeted Iraqi and Iranian pilgrims in the Shia holy city of Najaf on Saturday, killing at least three people, including two Iranians, and wounding more than 50, officials said.