Electoral reform group Bersih said it is shocked that yet another social activist has gone missing.
The latest case involves Peter Chong, a former aide to Subang MP R Sivarasa.
He is the fifth person involved in political or religious activities to have gone missing since November.
"Peter has been missing for more than two days. His son has reported to the police but to date there has been no news on his whereabouts.
"Indeed this is a very sad day for all Malaysians that disapperances are happening here in our country and yet there has been little assurance or news from the police on their efforts to locate the whereabouts of these missing persons," the Bersih steering committee said in a statement tonight.
The other four who have disappeared since November include pastor Joshua Hilmy and wife Ruth, Perlis Hope activist Amri Che Mat and pastor Raymond Koh.
"We fear for the safety and security of the five missing persons and demand that the police take greater efforts to be transparent and accountable by informing and updating on the progress of their search and rescue work.
"They must make this their priority as this helps mitigate some of the pains and anxiety suffered by the families of the disappeared," said Bersih.
The movement also called for a meeting with inspector-general of police on the matter.
Prior to Chong's disappearance, he had complained on Facebook about a stranger approaching him and warning him about people vanishing.
Chong recollected the "strange experience" with a young motorcyclist on March 31.
"He said he recognised me, that I like to go for protests and candlelight vigil.
I asked where is he from? I was a little worried and looked around but there were no one else around.
He said he supported what I do. He said 'but be careful... now there are many people suddenly disappearing. They know where everyone lives," said Chong.
Chong said when he queried who the man was, the man only told him to be savvy (pandai-pandai) before riding off.