Fidel Castro's failure to appear at the opening of his five-day birthday bash raised new concerns Wednesday about the ailing 80-year-old leader's health, but some supporters still hope he'll show at a huge military parade this weekend.

More than 1,000 foreign admirers gathered Wednesday morning for the start of an academic forum about Castro, and for some, the leader's absence thus far has been a letdown.

"To see Fidel Castro would have been really great," said 26-year-old Ecuadorean engineer Julio Crespo. "I came because I was really curious about Cuba."

But Crespo said he was a "realist," adding "it's known that his state of health isn't the best."

Nicaraguan politician Tomas Borge, however, said Castro's legacy was more important than his physical presence. The Cuban leader "doesn't live for just now, but forever," said the former Sandinista rebel leader.

A message said to be from Castro was carried live on state television Tuesday night, saying doctors had determined he wasn't well enough to attend the gala launch of the week's birthday festivities.

The message indicated Castro is far from recovered from a mysterious ailment that forced him on July 31 to turn over power to his brother, 75-year-old Defense Minister Raul Castro. The elder Castro's last public appearance was on July 26, when he delivered two speeches in eastern Cuba.

In the message, read to a crowd of 5,000 at the Karl Marx Theater, Castro expressed "the great pain of not having been able to personally give thanks and hugs to each and every one of you."

"I direct myself to you, intellectuals and prestigious personalities of the world, with a dilemma," said the note. "I could not meet with you in a small locale, only in the Karl Marx Theater where all the visitors would fit, and I was not yet in condition, according to the doctors, to face such a colossal encounter.

"My very close friends, who have done me the honor of visiting our country, I sign off with the great pain of not having been able to personally give thanks and hugs to each and every one of you," the note read.

The Cuban leader turned 80 on Aug. 13 but delayed his birthday celebrations as he recovered from surgery two weeks earlier for intestinal bleeding.

Castro wanted the delayed birthday celebrations held on Dec. 2 — this Saturday — to coincide with the 50th anniversary of the date that he and fellow rebels landed by boat in Cuba to launch their revolution.

Some supporters still hope Castro will appear at a big military parade planned for Saturday to celebrate the anniversary.

Rows of uniform-clad troops marched early Wednesday across the Plaza of the Revolution in rehearsal for the parade while Soviet MiG fighter jets whooshed overhead in a "V" formation. Crowds of curious Cubans gathered around massive anti-aircraft missile launches lined up just outside the square.

"I hope that on that day, during the parade, he will appear," elderly Arturo Munoz said of Castro during a brief interview in Old Havana. "He is needed because he is the guide of the revolution."

But construction worker Francisco Martinez said it was more important for Castro to get healthy than to show his face.

"I prefer that he get really well and that he assume his post, that he be in good health," said Martinez. "We need him to get better, not to be seen on a parade stand."

Whether Castro shows up or not, more than 1,300 politicians, artists and intellectuals from around the globe planned to honor the man who governed this communist-run island for 47 years.

Presidents Evo Morales of Bolivia and Rene Preval of Haiti have confirmed their attendance for the birthday festivities, along with former Ecuadorean President Rodrigo Borja and Nicaraguan President-elect Daniel Ortega.

Also expected are Argentine soccer great Diego Maradona, South African singer Miriam Makeba and Colombian Nobel laureate Gabriel Garcia Marquez. Nobel Peace Prize winner Adolfo Perez Esquivel, an Argentine human rights campaigner, was also expected.

Castro's good friend and political ally Venezuelan President Hugo Chavez won't be able to come as he is up for re-election on Dec. 3. But Chavez has promised to dedicate his electoral victory to Castro.

Cuban officials insist Castro is recovering, but U.S. officials say they believe he suffers from some kind of inoperable cancer and won't live through 2007. His ailment is a state secret.

Castro has been seen by the public only in photos and videos since he announced he was temporarily ceding power to his brother.

Along with the three-day academic conference that started Wednesday, admirers have planned a Friday night concert with Cuban and other Latin American artists, as well as an art exhibit.

More than 300,000 people are expected at Saturday's military parade.