Elsewhere, authorities announced the arrests of three people who they accused of planning similar attacks.
It was not immediately clear who was behind the attack at the District Courts complex in Quetta, the capital of Baluchistan province. At least 24 people were wounded in the suicide attack, said Jam Mohammed Yousaf, the top elected official the province.
He said a civil judge, five lawyers and some of the relatives of prisoners on trial were among those killed.
Information was not immediately available about who was on trial. The blast shattered windows and destroyed furniture inside the courtroom. Shoes, strips of clothing and body parts littered the scene.
Pakistan's Prime Minister Shaukat Aziz — who was visiting the northwestern city of Peshawar on Saturday to meet with families of some police officials who died in a separate suicide attack — condemned the bombing.
"It is an attack on humanity," he said.
Shortly after the attack, a crowd gathered outside the District Court complex and chanted anti-government slogans. Hundreds of relatives thronged a main government hospital where the dead and injured were taken. Rauf Khan, city police chief, said the situation was under control.
The latest attack came a day after police announced they had arrested five suspected militants from the southern city of Karachi and Rawalpindi, a garrison city near Islamabad, and that the suspects were planning suicide attacks on foreigners and minority Shiite Muslims.
Also Saturday, police in southern Pakistan said they arrested three Islamic militants who were planning suicide attacks to take place at forthcoming Shiite Muslim gatherings in Sindh province. The arrests were made late Friday, said Mazhar Sheikh, the police chief in Sukkur, a city about 300 miles northeast of Karachi, the provincial capital.
In Baluchistan province — made up of vast deserts — government forces have clashed with ethnic Baluch rebels in long-running unrest over political rights and royalties from rich natural gas fields.
Authorities in recent months have also arrested hundreds of suspected Taliban from Quetta and elsewhere as part of a campaign aimed at deporting Afghans living here without valid travel documents.
The conflict in the sparsely populated and impoverished region has drawn little attention from Western nations more concerned about Taliban militants believed to launch attacks from border regions of Baluchistan into Afghanistan, where NATO forces operate.
Humanitarian concerns emerged in the spring amid reports that tens of thousands of Baluch people had fled their homes in the volatile districts of Dera Bugti and Kohlu.
In August, a UNICEF survey counted 84,000 displaced and recommended to the provincial government that they needed help.
The latest attack came a day after police announced that they had arrested five suspected militants from the southern city of Karachi and Rawalpindi, a garrison city near Islamabad, and that the suspects were planning suicide attacks on foreigners and minority Shiite Muslims.