Tens of thousands of supporters of a Canada-based cleric, who has called for sweeping reforms and end to corruption in Pakistan, have descended on the capital, Islamabad, as part of a so-called "Long March".
Tahirul Qadri, who returned to his homeland last month after years in Toronto, has claimed that he will mobilise two million people, but security officials said that about 50,000 people had joined the much-publicised march on Monday.
Qadri has accused the government of corruption and incompetency, and said electoral reforms must be enacted by a caretaker administration before the country goes to polls in mid-May.
The cleric has said his followers will camp out until their demands are met.
"We will stay in Islamabad until this government is finished, all the assemblies are dissolved, all corrupt people are totally ousted, a just constitution is imposed, rule of law is enforced, and true and real democracy is enforced," he told the AFP news agency.
The significance of the rally will hinge on turnout, whether there is any violence and to what extent the protesters are able to penetrate Islamabad, where shipping containers have been used to seal off the main approaches.
Mainstream politicians fear that Qadri's demand for the military to have a say in the caretaker set-up could be a ploy by elements of the establishment to prolong the interim administration and delay elections.
In Islamabad, organisers set up a makeshift stage on Jinnah Avenue, where around 5,000 people waited for Qadri to arrive near parliament, which is barricaded behind shipping containers.
'Voice to masses'
His supporters say the cleric has given a voice to masses ruled by a feudal and industrial elite incapable of redressing a weak economy, a crippling energy crisis, insurgencies and sectarian violence.
"Look what we are witnessing in our country today. We have no gas, no power, no petrol. Is this the country we aspired to? We should give Qadri a chance," Huma Nadeem, a 20-year-old college student in Islamabad, said.
The floodlit platform has been festooned with portraits of Qadri and bulletproof glass has been put in place to protect him.
Thousands of security personnel have deployed with paramilitary soldiers, police and private guards searching all individuals before letting them cross scanner gates into the stage area, an AFP reporter said.
Mobile phone networks were shut down to stop fighters from detonating bombs as Pakistan suffers frequently from Taliban and al-Qaeda-linked violence.
The government says the Taliban have threatened to attack the gathering, though the Taliban has denied that charge.
Last Thursday, more than 90 people were killed in a suicide attack on Shia Muslims in the southwestern city of Quetta claimed by an extremist Sunni organisation.
If held on schedule, the election will mark the first democratic transition of power between two civilian governments in Pakistan's 65-year history, which has been marked by bloodless coups and extensive periods of military rule.