Hard-line religious protesters threw stones at a rally called to support five missing Pakistani activists on January 19 and demanded that the government file blasphemy charges against the missing men.
The liberal activists and bloggers, who had criticized political influence in the military and spoke up for religious minorities in Pakistan, all disappeared earlier this month.
Shortly after their disappearances, blasphemy allegations started to proliferate on social media and in complaints to the police. Convictions for blasphemy carry a death sentence in Pakistan.
As supporters of the activists called for their release at a rally in Karachi, about 100 members of a little-known religious group, Tehreek Labaik Ya Rasool Allah, started hurling stones and chanting slogans.
One of the zealots' banners read, "Beheading is the punishment of blasphemers."
The activists' supporters were forced to withdraw into a nearby building, and the religious demonstrators eventually were dispersed by police.
The five include university professor Salman Haider, known for criticizing the Taliban and the government, and Shi'ite activist Samar Abbas, who heads the Civil Progressive Alliance Pakistan.
No group has stepped forward to claim responsibility for the abductions. Government authorities say they are making every effort to find the activists.