PANAMA CITY, Panama - The anniversary of the 1989 U.S. invasion was declared a day of "national mourning" by Panama's legislature yesterday, and it established a commission to determine how many people were killed when U.S. troops stormed the capital.

The measure was unanimously approved as Panama commemorated the 18th anniversary of the day thousands of U.S. soldiers landed to arrest dictator Manuel Noriega on drug charges.

"This is a recognition of those who fell on Dec. 20 as a result of the cruel and unjust invasion by the most powerful army in the world," said Rep. Cesar Pardo of the governing Democratic Revolutionary Party, which holds a majority in the legislature.

The measure, which requires the approval of President Martin Torrijos, calls for a monument to honor the dead, most likely in El Chorrillo, a neighborhood destroyed by bombs during the attack.

U.S. officials downplayed the issue. "We prefer to look to the future," U.S. Embassy spokesman Gavin Sundwall said. "We are very satisfied to have a friend and partner like Panama, a nation that has managed to develop a mature democracy."

Polls at the time indicated that Panamanians overwhelmingly welcomed the invasion that rid them of Noriega, who was later convicted in absentia here and sentenced to 60 years in prison on charges of embezzlement, corruption and murdering opponents.

But national feelings have grown that the invasion was a blow to the nation's dignity.

The government estimates that 472 to 500 Panamanians were killed. Human-rights groups say 1,000 died. About 25,000 U.S. troops invaded, 23 of whom were killed.

Yesterday's measure establishes a "truth and reconciliation" commission with representatives of the Roman Catholic Church and Attorney General's Office to determine the exact number of civilian and military deaths.

It also will try to list the names of those killed from October 1968, when military rule began in Panama under the current president's father, to December 1989, when Noriega was ousted.

Noriega became president after his mentor, Gen. Omar Torrijos, died in a 1981 plane crash.

Noriega, a former collaborator of the CIA, was sentenced to 30 years on U.S. drug-trafficking charges in 1992. His sentence, reduced for good behavior, ended on Sept. 9, but he remains in custody until the resolution of an extradition request by France, which wants to try him on money-laundering charges.

While Noriega was imprisoned, Torrijos' old party was voted out of power and the military was replaced by a national police force.

Torrijos' son Martin, who earned an economics degree at Texas A&M University, worked to distance himself from the old dictatorship, noting he was in the United States at the time. He was elected president in 2004.

"All political sectors have wanted to cast a blanket of forgetfulness" over the invasion, Rep. Pedro Miguel Gonzalez, president of the legislature since September, said on the eve of the anniversary.

"Maybe out of shame or for other reasons, but it isn't fair to all of those Panamanians that still lie in common graves," he said.

The United States has a pending case against Gonzalez for the alleged murder of U.S. soldier Zak Hernandez in 1992 during an ambush that came just days before a visit from President George H.W. Bush.