(CNN) -- Six years after the worst terror attacks on U.S. soil, three in 10 Americans believe the United States and its allies are winning the global war on terror -- one of the main justifications cited by the Bush administration for the war in Iraq.

An American flag hangs from the Pentagon on Tuesday, the sixth anniversary of the September 11 attacks. more photos »

According to a CNN/Opinion Research Corp. poll conducted Friday to Sunday, about half of the 1,017 adult Americans questioned said they believe neither side is winning the war on terror, while 19 percent said the terrorists have the upper hand. The poll has a sampling error of plus or minus 3 percentage points.

A separate poll conducted August 6 to 8 found that 61 percent of the 1,029 adult Americans questioned said they were not satisfied with the way things were going in the war on terror, and 38 percent felt safer from terrorism than before the 9/11 attacks.

Thirty-two percent of the respondents said they felt less safe than before the attacks, and 29 percent felt as safe now as then. The poll also had a sampling error of plus or minus 3 percentage points.

Nearly 3,000 people were killed September 11, 2001, when hijacked planes slammed into the World Trade Center's twin towers, the Pentagon and a field in Pennsylvania. The attacks shocked a country that had largely felt insulated against terrorism, at least on its home turf.

Six years later, 57 percent of those questioned in the September poll believed terrorists would find a way to launch attacks no matter what the U.S. government did. Forty percent said they wouldn't. Watch how this year's poll results compare to past years »

Nearly two-thirds of those questioned said the United States would never return to normal, while 30 percent said eventually, things would go back to the way they were.

Across the country Tuesday, various cities held ceremonies or moments of silence to mark what President Bush has called the day that changed America forever.

The president and first lady Laura Bush as well as Vice President Dick Cheney and his wife, Lynne, participated in a moment of silence on the White House South Lawn at 8:46 a.m., the moment the first plane hit the World Trade Center's north tower.

Secretary of State Condoleezza Rice, FBI Director Robert Mueller, members of the House of Representatives and Senate, and other dignitaries also took part. Most wore black or other dark colors under a sky of cloudy gray. See how people around the world are marking the anniversary »

Earlier, the president attended a service at St. John's Episcopal Church in Washington.

In New York, bells began ringing at St. Patrick's Cathedral at 8:46 a.m. to honor victims of the attacks.

A slow parade of police, firefighters and members of the Port Authority of New York and New Jersey marched to ground zero, the site of the fallen towers, accompanied by the droning of bagpipes.

The New York Fire Department lost 343 members, the New York Police Department 23 officers and the Port Authority 37 people in the attacks.

After a moment of silence, led by Mayor Michael Bloomberg, and the tolling of a bell, family members and friends began reading the names of the 2,603 victims who died at the World Trade Center. Rain rippled the surface of a reflecting pool at the ground zero memorial, where people placed flowers along the edges.

The New York Stock Exchange held a moment of silence before the opening bell at 9:29 a.m.

At 9:38 a.m., a ceremony was held at the Pentagon to mark the moment when a hijacked plane hit the building, killing 184 people.

In brief remarks before the wreath-laying ceremony, Defense Secretary Robert Gates said the United States would stop at nothing to defend its citizens and values.

"Let there be no doubt that anyone wishing to revisit harm upon this country will find, in the men and women of this department, adversaries who have found clarity of purpose in their grief, a strength of resolve in their anger," Gates said.

"The enemies of America, the enemies of our values and our liberty, will never again rest easy, for we will hunt them down relentlessly and without reservation."

Near Shanksville, Pennsylvania, a steady rain fell as dozens participated in a memorial ceremony for the 40 passengers and crew aboard United Airlines Flight 93, which crashed into a field after passengers attempted to wrest control of the aircraft from four hijackers. E-mail to a friend

All About Terrorism • George W. Bush • September 11 Attacks