Hundreds of protests and vigils are being held across the US to mark the 2,000th military death in Iraq.
President Bush said more sacrifices would need to be made
In Washington, the Senate held a minute of silence, and the names of the dead were read out on the floor of the House of Representatives.
President George W Bush has warned Americans to brace themselves for more casualties, saying no-one should underestimate the difficulties ahead.
Unofficial estimates put Iraqi civilian deaths since the war at about 25,000.
Staff Sgt George Alexander, 34, wounded by a bomb in the insurgent stronghold of Samarra earlier this month, became the 2,000th US death when he died of his wounds on 22 October.
Anti-war protesters say they will stage more than 300 protests at war memorials and federal buildings across the country.
Activist Cindy Sheehan, mother of a US soldier killed in Iraq, said she planned to stage a civil disobedience protest outside the White House in a bid to be arrested.
She urged opponents of the war to stage their own protests: "Go to your senators' offices, to federal buildings. Sit down and say enough is enough. The killing has to stop sometime."
In a speech on Tuesday to officers' wives, President Bush acknowledged the deaths, saying "each loss of life is heartbreaking".
But said the best way to honour the dead troops was to "complete the mission and lay the foundation of peace by spreading freedom".
He said it was a "dangerous illusion" to claim that the US would be better off if it left Iraq now.
"This war will require more sacrifice, more time and more resolve," he said.
A recent Ipsos/AP poll suggested that public support for the president is at an all-time low of 39% for two straight months.
But the US armed forces have continued to enjoy a good reputation at home, where public opinion has also remained remarkably and strikingly tolerant of American casualties, the BBC's defence correspondent Rob Watson says.
That said the deaths in Iraq are taking their toll on recruitment with all branches of the US military struggling to attract new men and women.
A spokesman for the American-led multinational force in Baghdad appealed to media not to make too much of the 2,000 figure.
"It is an artificial mark on the wall set by individuals or groups with specific agendas and ulterior motives," Lt Col Steve Boylan told AP news agency.
According to a count made by UK-based academics and peace activists this summer, nearly 25,000 civilians died violently in the first two years of the conflict while nearly 43,000 were injured.
One of the groups involved, Iraq Body Count (IBC), says on its website the toll could now be more than 30,000.
While many of the deaths between March 2003 and March 2005 are attributed to militant attacks and violent crime, IBC says US-led forces killed 37% of civilian victims.