Iraqis lined up amid tight security Thursday to vote in a historic parliamentary election the U.S. hopes will lay the groundwork for American troops to withdraw, with a mortar landing near the heavily fortified Green Zone just minutes after polls opened.

No injuries were reported, but the blast underscored concerns of violence despite a promise by Sunni insurgent groups not to attack the polls.

Dozens of Iraqis waiting to cast ballots at Baghdad's city hall went through three separate checkpoints as police searched each person entering the downtown site.

"The first voting process to choose a parliament with a four-year term in Iraq has started," senior election official Abdul-Husein Hendawi said.

He said some polling stations in Ramadi, an insurgent stronghold 70 miles west of Baghdad, had not yet opened for security reasons.

A large explosion hit downtown Baghdad within minutes of the polls opening. Police said it apparently was caused by a mortar landing near the Green Zone that houses the Iraqi government and the U.S. and British embassies, but no injuries were reported.

A coalition of Shiite religious parties, which dominate the current government, was expected to win the largest number of seats — but not enough to form a new administration without alliances with rival groups.

Outside Baghdad city hall, Abbasiya Ahmad, 80, said she had voted for the governing Shiite United Iraqi Alliance.

"We hope that they will bring us security and safety. Also they are clerics, and clerics do not steal our money. We want people who protect our money," she said.

The Bush administration hopes the new parliament will include more Sunni Arabs to help establish a government that can lure other Sunnis away from the insurgency. Such a development would make it possible for the United States and its partners to start to draw down their troops next year.

But there were signs of the sectarian tensions that threaten the nation's future and the Bush administration strategy: Angry Shiites protested what they considered to be a televised slur on the country's religious leadership, and rumors spread of ballots smuggled in from Iran.

Also Wednesday, police arrested two suspected insurgents carrying 72 bombs, police Lt. Col. Ahmed Hajoul said. He said the pair said they planned to hide the bombs in the largely Shiite city of Hillah to explode when the polls opened.

A loud explosion also was heard in central Baghdad shortly before 1 a.m. Thursday, hours after police said several mortar shells exploded in southwest Baghdad, setting some shops on fire.

Rumors also swept the Iraqi capital early Thursday that the water supply had been poisoned after warnings against drinking tap water were broadcast through mosque loudspeakers, but they were quickly denied by the Health Ministry.

Up to 15 million Iraqis were to choose 275 members of the new parliament from among 7,655 candidates running on 996 tickets, representing Shiite, Sunni, Kurdish, Turkomen and sectarian interests across a wide political spectrum. Iraqis do not vote for individual candidates, but instead for lists — or tickets — that compete for the seats in each of the 18 provinces.

Some preliminary returns were expected late Thursday, but final returns could take days if not weeks.

President Jalal Talabani, a Kurd, cast his ballot in the northern city of Sulaimaniyah.

"This is a good day and the Iraqi people bear the responsibility to vote for a better future. I hope that the Iraqi people will stay united. We hope that the people will vote to keep the constitution that was approved by the Iraqi people," he said.

Election of the new parliament, which will serve a four-year term, marks the final step in the U.S. blueprint for democracy. The vote will cap a process that included the transfer of sovereignty last year, selection of an interim parliament Jan. 30 and ratification of the constitution in October. The new parliament will name a government, including a new prime minister.

"In spite of the violence, Iraqis have met every milestone," President Bush said in Washington.

For the Bush administration, the stakes are nearly as high as for the Iraqis. A successful election would represent a much-needed political victory at a time of growing doubts about the war among the American public.

"We are in Iraq today because our goal has always been more than the removal of a brutal dictator," Bush said. "It is to leave a free and democratic Iraq in its place."

U.N. Secretary-General Kofi Annan also urged the Iraqi people to vote, saying they have another "historic opportunity" to shape the country's political future.

Insurgent threats and boycott calls kept many Sunnis at home in the January election despite a national turnout of nearly 60 percent. That enabled Shiites and Kurds to dominate the current legislature, sharpening communal tensions and fueling the insurgency.

This time, more Sunnis Arabs were in the race and changes in the election law to allocate the majority of seats by district all but guaranteed strong Sunni representation.

More than 1,000 Sunni clerics called on their followers to vote, and insurgent groups, including Al Qaeda in Iraq and the Islamic Army in Iraq, pledged not to attack polling stations even though they oppose the political process.

Nevertheless, tens of thousands of Iraqi soldiers and police were guarding polling stations, with U.S. and other coalition forces standing ready in case of trouble. U.S. troops and bomb-detecting dogs checked thousands of polling stations before handing over control to Iraqi police.

As a sign of Sunni interest, mosques, walls, houses and lamp posts in Baghdad's Sunni district of Azamiyah were festooned with posters of Sunni candidates. In January, few people in Azamiyah voted and some polling stations didn't even open.

U.S. officials were optimistic about a heavy turnout in Fallujah, the Sunni insurgent stronghold captured by American forces last year. Campaign posters were plastered Wednesday over blast walls along the street, at police checkpoints and on the walls of houses.

"In January, turnout was low. In the referendum it was tremendous and tomorrow it will be better," said John Kale Weston, U.S. State Department spokesman in the city.

Still, U.S. officials warned that a successful election alone will not end the insurgency. Also needed is a government capable of reconciling Iraq's disparate groups.

The Americans also were eager to avoid protracted negotiations to choose a new prime minister and Cabinet — a process that dragged on for three months after the last vote.

"I think the elections are a positive step, but it will not be enough to ensure stability," U.S. Ambassador Zalmay Khalilzad told Al-Jazeera television. "There should be a good government that represents all Iraqis, and the security forces also should be formed by all Iraqi sects."

His comments about the security forces referred to Sunni Arab complaints that the Shiite-dominated army and police have abused Sunnis. On Tuesday, Khalilzad said at least 120 abused prisoners had been found in two detention centers run by the Interior Ministry since November.

On the eve of the election, sectarian tensions swelled over what Shiite political parties considered an offensive remark made by an Iraqi Shiite panelist on Al-Jazeera. Fadel al-Rubaei said Shiite clerics should not participate in politics, and he accused them of conspiring with the Americans against the mostly Sunni insurgents.

The statements angered many Shiites, including many who did not see the Al-Jazeera broadcast but saw reports about it on an Iraqi station, Al-Furat, owned by the biggest Iraqi Shiite party, which used the report to fire up its supporters.

Hours later, thousands of people chanted anti-Al-Jazeera slogans in the streets of the Baghdad neighborhoods of Sadr City and Karradah, and in major cities throughout the Shiite south.

In Nasiriyah, Shiite protesters set fire to a building housing the offices of former interim Prime Minister Ayad Allawi, a secular Shiite, and the Iraqi Communist Party.

Officials at the Qatar-based Al-Jazeera were not available for comment. But Baghdad correspondent Atwar Bahjat told The Associated Press she resigned from her job "in protest of what the guest of the station said."

Rumors swept Baghdad that a tanker truck filled with thousands of blank ballots had been smuggled into the country from Shiite-dominated Iran. Many Sunnis consider Shiite political parties as agents of Iran.

The Interior Ministry denied any attempt to smuggle ballots, and the election commission said the only trucks in the area were its own delivering election materials to polling stations.