BAGHDAD (Reuters) – Iraqis began voting on Sunday in a parliamentary election that Sunni Islamist insurgents have vowed to derail in an effort to plunge the war-shattered country back into sectarian bloodshed as United States troops leave.

Scattered explosions occurred as polling stations opened at 7 a.m. (11 a.m. Sunday in Manila). At least three mortar rounds landed near voting centers in the town of Baiji, 180 kilometers north of Baghdad, wounding three people, police said.

Several blasts from mortar rounds, rockets or bombs echoed over the capital, including the Green Zone, a fortified government and diplomatic enclave, but police reported no casualties.

Iraq’s political course will be decisive for President Barack Obama’s plans to halve US troop levels over the next five months and withdraw entirely by end-2011. It will also be watched closely by energy companies that have committed themselves to investing billions in Iraq’s vast oilfields.

Voters in the ethnically and religiously divided country can pick between mainly Shi’ite Islamist parties that have dominated Iraq since Saddam Hussein’s fall and their secular rivals.

“This election marks another step in the march of our democracy – and also a test,” said President Jalal Talabani, a veteran Kurdish politician seeking another term.

No bloc is expected to win a majority, and it may take weeks or months to form a government, risking a vacuum that armed groups such as Iraq’s al Qaeda offshoot might exploit.

Few elections in the Middle East have been as competitive as this one. Its conduct could determine how democracy in Iraq affects a region used to kings and presidents-for-life.

About 6,200 candidates from 86 political groups are vying for 325 parliamentary seats.

Shi’ite Prime Minister Nuri al-Maliki, whose State of Law coalition is claiming credit for improved security since the peak of sectarian warfare in 2006-07, faces a challenge from one-time partners looking to recapture Shi’ite support.

He also takes on a secular list tapping into exasperation with years of conflict, poor public services and corruption, and hoping to gain support from the Sunni minority once dominant under Saddam’s rule.

Former Prime Minister Iyad Allawi, a secular Shi’ite who heads the cross-sectarian, secularist Iraqiya list, is already complaining about irregularities in early voting.

“Iraq, which they wanted to turn into a theater of crimes, has become a theater of democracy, elections and freedom,” Maliki said on the eve of the vote.

Last week 600,000 people, including soldiers and detainees, voted early. Up to two million Iraqi expatriates and refugees abroad can also vote.