Cricketer-turned-politician Imran Khan has begun a motorcade "march" to Pakistan's restive tribal areas to protest against US drone strikes.
The two-day protest started in Islamabad and is due to end in South Waziristan.
Mr Khan set off with a few hundred people. It is not clear if authorities will allow the march to reach its goal.
Mr Khan, like many Pakistanis, says the attacks kill large numbers of civilians and foster support for militants.
"No-one should be allowed to be judge, jury [and] executioner," Mr Kahn said before setting off.
"It's totally counter-productive. All it does is it helps the militants to recruit poor people. Clearly if they were succeeding, these drone attacks, we would be winning the war. But there's a stalemate."
US officials insist strikes by the unmanned aircraft rarely claim civilian casualties and are an effective weapon against the Taliban and al-Qaeda.
Mr Khan expects large numbers to join him, but the BBC's Orla Guerin in Islamabad says it is unclear how far the convoy of vehicles will be able to proceed.
Pakistani authorities have expressed security concerns and they may stop the march before it reaches the tribal areas, she says.
The Taliban have criticised both Mr Khan and the rally, but the politician told the BBC he was not worried about militant attacks.
In Dera Ismail Khan, the city in which participants were expected to stay the night, the Taliban distributed leaflets saying it would "welcome" them with bombs.
About 80 western peace activists are in the protest convoy.
· Recent US report highlighted "terror" felt by civilians in north-west Pakistan, where drones target areas such North and South Waziristan
· Hundreds of low-level militant commanders and substantial minority of civilians killed
· Exact figures difficult to compile because independent media and researchers denied access to area by authorities
· Bureau of Investigative Journalism estimates 2,570-3,337 people killed in drone strikes, of which 474-884 were civilians
· Living Under Drones report says top commanders account for estimated 2% of victims
Mr Khan's Pakistan Tehreek-e-Insaf (PTI) party had said the march and a rally in South Waziristan would attract hundreds of thousands of people.
Mr Khan's critics accuse him of trying to boost his party's popularity but supporters say the march shows he is in touch with the concerns of Pakistani people.
Mohammad Ansar Adnan, a student in Islamabad, told Reuters news agency that drone attacks were "an escalating problem".
"If Imran Khan is taking a step to resolve this issue, I think we should all go along with him, and once we are there, we should offer prayers for peace."
Authorities in South Waziristan say they have not given the PTI permission to stage a rally and they cannot provide security for so many people. Officials on the border of the tribal areas told BBC Urdu on Saturday that they would not allow Mr Khan to enter.
The government of Pakistani President Asif Ali Zardari says drone strikes are counterproductive and a violation of its sovereignty.
However, it has done nothing to stop them and many Pakistanis - including Imran Khan - believe this amounts to tacit consent.
In September, a report by Stanford and New York Universities in the US said Pakistani civilians were being "terrorised" 24 hours a day by CIA drone attacks.
It said rescuers treating casualties were also being killed and wounded by follow-up strikes.
The scale of civilian deaths has been difficult to assess because independent media and researchers are denied access to the tribal areas.
US President Barack Obama has insisted that the drone strategy is "kept on a very tight leash" and that without the attacks, the US would have had to resort to "more intrusive military action".
Source: BBC News