No one has claimed the attack, which injured dozens of people, police said.

Quetta is the capital of Balochistan province, where for several years nationalist militants have been fighting for greater autonomy.

A severed head, thought to be the bomber's, was found in the aftermath of the attack, police said.

"The bomber entered the courtroom, stood leaning against a wall and detonated, " Baluchistan police chief Tariq Khosa told a news agency.

Several of the injured are in a serious condition with severe burns.

Pakistani Prime Minister Shaukat Aziz said the attack was against the Muslim faith and against humanity.

Pakistan's security forces are already on high alert, following three suicide bomb attacks in the region in recent weeks.

Despite the government's denials, there are regular reports of Taliban insurgents operating in the province, which borders Afghanistan, our correspondent says.

Pakistan is under pressure from its western allies who want to see the country doing more to control the highly volatile situation along its western borders.