The actor passed away Monday following a 2-year battle with pancreatic cancer ... according to his Facebook page, which has been flooded with an outpouring of fans, friends and family honoring the late actor.

Camden's partner, Bea Henderson, said he was diagnosed in February last year "after months of mysterious pain." She added, "His condition has declined a lot in the last 2 months and tremendously in the last several days," noting he was unable to communicate toward the end.

She says Camden decided "for many reasons" to keep his illness a secret ... and admits she's sorry for the big shock that's now coming to fans.

Actor Doug Jones, who worked alongside Camden on 'Buffy,' wrote, "So many memories including working together, decades of laughing, sharing mutual friends, get togethers, travel, conventions, and huggles with each other and our fans."

Apart from playing one of the Gentlemen in the vampire-slaying series, Camden appeared in various projects like "Angel," "The Bay," "Average Joe," "Shameless," and "The Mentalist."

He was 68.