Islamabad  - Street protests erupted in south-western Pakistan on Thursday after police found the bodies of three missing nationalist leaders, officials said.

Groups of angry protestors blocked roads and stoned buildings and vehicles in different towns in the restive Balochistan province in a reaction to the killings.

A police patrol discovered the mutilated bodies of the Baloch leaders before dawn near the town of Turbat, Ayaz Baloch, a senior police officer, said.

A Baloch lawmaker, Senator Mir Hasil Khan Bizenjo, told reporters in the provincial capital Quetta that the trio had been allegedly picked up by security officials Friday and killed the same day.

"When the intelligence agencies can go to this extent, the question arises for Baloch people whether to take up arms [against the state]," Bizenjo said.

He warned of a violent reaction "if the spying agencies continue to suppress the Balochs through use of force."

A police official was killed when rioters opened fire in Khuzdar, a town located around 400 kilometres north-east of Turbat, police said.

The protestors torched at least five vehicles, including a mini-truck of the United Nations, in Quetta.

Television footage showed riot police firing tear gas canisters to disperse the violent crowds.

Markets in the troubled areas were shut down, while authorities also closed educational institutions in the wake of the violence.

Balochistan is Pakistan's largest but most sparsely populated province where ethnic tribesmen are waging a sustained low-level insurgency to seek political autonomy and greater share in profits from natural resources exploited in the area.

Government forces have long been accused of resorting to extrajudicial killings and illegal detentions to quell the insurgency.

A shadowy ethnic group, the Balochistan Liberation United Front, had kidnapped an American UN aid worker, John Solecki, and freed him as "a goodwill gesture" on Saturday after two months of detention.

The abductors had sought the release of 1,109 men and 141 women, who they said were being held by the security forces. (dpa)