Tribal leader Nawab Akbar Bugti has been killed in a battle between tribal militants and government forces in Balochistan province, Pakistan says.
Nawab Bugti wanted autonomy for Balochistan
The battle near his mountain hideout in south-west Pakistan caused heavy casualties, reports say.
At least five soldiers and at least 30 rebels are thought to have died.
A 24-hour curfew has been imposed in the city of Quetta where hundreds of students from Balochistan university rioted at news of the death.
The octogenarian was at the head of a tribal campaign to win political autonomy and a greater share of revenue from Balochistan's gas reserves.
The BBC's Dan Isaacs reports from the Pakistani capital, Islamabad, that his death represents a major victory for the government in its campaign to undermine rebels in Balochistan.
"It is confirmed, Nawab Bugti has been killed in an operation," Information Minister Mohammad Ali Durrani told Reuters news agency.
The battle reportedly took place near the town of Dera Bugti.
One report said government forces had targeted between 50 and 80 rebel fighters, after being led to the area by an intercepted satellite phone call.
Troops launched the operation three days ago and swooped on the rebels in helicopters, one security official in the provincial capital, Quetta, told Reuters.
Other officials told the AFP news agency that Pakistani forces launched air strikes against a cave complex on the border of Dera Bugti and Kohlu districts on Friday, before special forces moved in on Saturday.
However, there was no official confirmation that Mr Bugti's body had been found in the cave where the fighting was focused.
Some government sources suggested that the real death toll among the security forces was closer to 20.
Balochistan is Pakistan's biggest province, and is said to be the richest in mineral resources. It is a major supplier of natural gas to the country.
But for decades, Baloch nationalists have accused the central government in Islamabad of depriving the province of its due.
They say the government took away income from natural gas and other resources, while spending only a trivial amount on the province.
Nawab Akbar Bugti - known to many as the Tiger of Balochistan - played a major role in the politics of the province for more than five decades, the BBC's Steve Jackson writes.
Sometimes he pursued his nationalist agenda from positions of authority and sometimes as a rebel leader.
He served in the federal government and was on occasion governor and chief minister of Balochistan.
The latest fighting between government forces and Mr Bugti's followers began after attacks by separatists on the gas infrastructure in the region.
'I don't bow'
At the age of 79, Mr Bugti was forced to leave his tribal land and go into hiding after his house was bombed.
In one of his last interviews - with the BBC's Urdu Service in July this year - he was asked why a peace deal between his tribes and the government had not been implemented.
"They say that I am intransigent, I don't listen to them, I don't bow before them," he said.
"They say that I should bow before them and salute them, and give up my weapons, and then everything will be all right."
His vision for Balochistan has never been achieved but the insurgency he led has been one of the biggest headaches for President Pervez Musharraf in recent years, our reporter writes.
The main question now is whether or not his death will provoke more violence from the separatists, he adds.