Hisham of Global Voices sets the scene:
[Muammar al-Gaddafi] is the longest serving of all current non-royal national leaders in the world. Hoping to emulate recent popular uprisings in Egypt and Tunisia, Libyan pro-democracy activist have been calling for protests against the 41-year-old autocratic rule of Gaddafi. They set February 17 as a “Day of Rage”, using social networking websites to convince millions to take to the streets and peacefully call for change. But it seems that Libyans are too eager to voice their rage and anger at their leader as many decided to demonstrate today.
Gaddafi is already fretting Facebook's impact. Amira Al Hussaini provides more background by translating blogger Mohammed Maree:
[The protesters] are the relatives of the martyrs slain in the Abu Slim prison massacre, who were joined by scores of other supporters. They are demonstrating against the arrest of the official spokesman on behalf of the families, who was arrested by the Libyan security forces, for no reason.
The Abu Slim prison massacre happened on June 29, 1996, when a security group close to the Libyan dictator's regime broke into the prison, and mowed down not less than 1,200 political prisoner, who had objected then to their inhumane conditions inside the prison. This bloody operation continued for three hours, and the victims were then buried in the prison's courtyard and in mass graves in Tripoli. This horrendeous crime was only revealed in 2008, when information was leaked to the families of the martyrs, and human rights defenders inside and outside Libya.