U.S. citizens on Thursday marked their annual Thanksgiving Day holiday — a centuries-old harvest time tradition.

Millions were on the road or in trains or airplanes, making this holiday one of the busiest times for travel in the United States.

U.S. President Barack Obama made telephone calls to some of the American troops in Iraq, who are celebrating their last Thanksgiving there since the U.S.-led invasion in 2003. All of the fewer than the 20,000 troops remaining in Iraq are scheduled to be withdrawn by the end of the year.

In a radio address the president asked Americans to remember those who are spending the holiday overseas. He also urged U.S. citizens facing difficult economic times to believe in the nation's ability to meet its challenges. He said the nation's problems did not develop overnight and will not be solved overnight.

Mr. Obama thanked those who are taking time out to do volunteer work, such as serving Thanksgiving Day meals at homeless shelters.

Thanksgiving Day has a long tradition in the United States. In 1863, President Abraham Lincoln issued a proclamation calling for the last Thursday of November to be observed as a day of thanks.