Colleagues of Pakistani activist Samar Abbas say that he is missing under unknown circumstances.
The Civil Progressive Alliance, which Abbas heads, said on January 12 that Abbas disappeared in the port city of Karachi on January 7.
Abbas's disappearance comes amid a spate of similar incidents in which four liberal bloggers were reported missing in several different cities between January 4 and January 7.
The Human Rights Watch monitoring group and the United Nations have expressed concern over the disappearances.
"No government should tolerate attacks on its citizens," David Kaye, the UN's special rapporteur on freedom of expression, was quoted as saying.
Talib Raza of the Civil Progressive Alliance said Abbas's disappearance seemed to be part of "an organized attempt to shut the progressive and liberal voices in the country."
Pakistan's Interior Ministry has said it is investigating the disappearances.