In the Lao capital, a market fair was planned Sunday by Sombath's wife, Singaporean Ng Shui Meng, to commemorate her husband's disappearance, Focus Global South officer Shalmali Guttal said at the Bangkok protest.
Protests are strictly prohibited in communist Laos.
Ms Ng on Sunday issued a letter to Mr Sombath, which was read out in front of the Lao embassy in Bangkok, where about 20 representatives of the Asia-Europe People's Forum, Mekong Youth Network and Japan-based civil society organisations had gathered.
"Your leaders, including President Choumaly Sayasone, have promised to make a serious investigation and find the perpetrators who took you," Ms Ng wrote. "I can only hope that their promise is true, even though it has been a year and there is no information whatsoever on your whereabouts or your situation," the letter said.
Mr Sombath, 61, went missing on Dec 15, 2012, after being detained at a police checkpoint outside the Lao capital. CCTV images captured him leaving his vehicle, then getting into a pickup truck and being driven away.
Laos' communist regime has offered no explanation for Mr Sombath's disappearance, suggesting it may have resulted from a personal dispute.
The local press in Laos is tightly controlled and civil society activism has been described as "fledgling" by international rights groups.
"He is a civil society leader in Laos where civil society is just emerging," Ms Ng told a recent forum in Bangkok. "He always told me that he feared the publicity exposed him to too much risk."