Hayden Panettiere Death: Actress Alyson Stoner Posts Heartbreaking Tribute

Hayden Panettiere's colleague Alyson Stoner broke down in tears after learning of her death ... sharing a video on social media paying tribute to her fellow "Kingdom Hearts" star.

Alyson was very emotional in the short clip she posted Monday ... telling fans she knows she looks rough at the moment -- but asking for them to give her grace as she talks about her late friend.

She says she worked with both Hayden and Hayden's late brother, Jansen Panettiere ... and both deaths have been heartbreaking.

Alyson then says, "From one Kairi to another, I just wanted to acknowledge the tremendous loss. Not only for the 'Kingdom Hearts' family, but, of course, for all of the close loved ones and millions of lives that Hayden reached."

Kairi is one of the main characters in the popular video game franchise. Hayden voiced Kairi in the first few editions of "Kingdom Hearts" before Alyson took over the character.

Alyson ends her message by telling her fans and followers to take care of themselves during this trying time.

As you know ... Hayden was pronounced dead on Sunday afternoon in South Carolina after first responders were called to a residence for a woman suffering from cardiac arrest.

EMS found an unresponsive Hayden and performed "advanced cardiac life support measures," but they were unable to resuscitate her.

Dispatch audio mentions a possible overdose ... and we've confirmed Narcan, a medication used to reverse the effects of an opioid overdose, was found in the apartment where she died.

The cause and manner of death remain pending further investigation.