"Does it have to be your own daughter for you to understand? She could've been mine," he tweeted.
Hassan, founder-president of Makkal Needhi Mayyam, said he felt "angry" as a man, father and as a citizen for "failing" the minor girl.
"I am sorry my child, We didn't make this country safe enough for you. I'll fight for justice at least for future kids like you. We mourn you and won't forget you," he added.
A nation-wide outrage broke over the rape and killing of the minor girl in Kathua, with Congress leaders, including its president Rahul Gandhi, staging protests over the incident and mourning her death.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)