Mali, the "world's saddest elephant," has passed away following health complications after decades of confinement at Manila Zoo in the Philippines ... where she spent most of her life alone in a pen.

The announcement was made Wednesday by the city's mayor, Honey Lacuna, during a news conference -- a day after the zoo's chief veterinarian, Dr Heinrich Patrick Peña-Domingo, confirmed the captive elephant had cancer in some of her organs and a blockage in her aorta.

The diagnosis came after Mali was seen Friday repeatedly rubbing her trunk against a wall -- an indicator she was in pain -- and then laying on her side and breathing heavily Tuesday, passing away later in the day.

At only 11 months old, Mali, whose full name was Vishwamali, was gifted to former Philippines first lady Imelda Marcos by the Sri Lankan government in 1981.

In her early years, she lived at Manila Zoo alongside another elephant, Shiva, who died in 1990 after arriving in 1977.

Since then, Mali had been the zoo's only elephant and lived in solitary until her death.

Animal rights activists have long criticized the conditions of Manila Zoo ... blasting the keepers for being ill-equipped to provide the animals with proper care.

In 2013, Paul McCartney wrote to Philippine President Benigno Aquino III begging for Mali to be moved to an animal sanctuary to "bring an end to her suffering," ... while animal welfare group PETA added at the time that Mali "endures intense confinement, loneliness, boredom and isolation" at the zoo.

Pamela Anderson and Dr. Jane Goodall have backed the "Free Mali" movement.