The 26-year-old vice-chairman of the Civic Party's Kowloon East branch was found hanged in Tai Po yesterday, five days after he went missing.
The party expressed shock and grief at the death of Eric Tsui Kwok-fung. He had joined the party in 2007 and been active in its youth wing, Youth Civics.
Police found his body at about 2pm on a slope near Villa Castell, Tai Po Road, after a search.
Tsui had worked as a teaching assistant at Buddhist Hung Sean Chau Memorial College in Wong Tai Sin.
Kowloon West police superintendent Tan Wing-yuen said the search began after a resident told police early yesterday that he thought he saw a man fitting Tsui's description in Deerhill Bay, next to Villa Castell, on Tuesday - the day Tsui went missing after leaving his Sham Shui Po home.
Police said they found no suicide note but that Tsui's identity and Octopus cards were on his person when they discovered him.
Upon being informed, grieving relatives and party members including lawmaker Claudia Mo Man-ching rushed to the scene.
"Today, we lost a friend," a statement released by the pan-democratic party said.
Party leader Alan Leong Kah-kit said all party members were distressed and mourning.
"It has come as such a shock ... Even those of us who worked closely with Eric did not sense anything wrong, with his teaching career or his health.
"We don't know of anything that would suggest he had financial difficulties either. Maybe there was something he hid from us so much so that none of us knew about it."
Tsui, a graduate of Chinese University's government and public administration department, helped Tanya Chan in 2008 when she ran for a Hong Kong Island Legislative Council seat.
"Eric was a very active member and outspoken in many of our public policy debates," said Leong, who visited Tsui's home on Friday and met his parents.
A plan to open a bank account to receive donations for Tsui's parents was dropped after the couple said they had no need for funds.
Similarly, a plan to mourn Tsui's passing at the party's North Point headquarters tonight was cancelled after they expressed a desire to keep the grieving private and low key.