Tens of thousands of Pakistani protesters demanding sweeping reforms poured into Islamabad late Monday, led by a cleric accused of trying to sow political chaos ahead of elections.
He claims to be able to mobilise two million people and says his followers will camp out with food, fuel and blankets until their demands are met.
But 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.
In the late afternoon, security officials told AFP that the crowd had swollen to around 50,000 people. Observers suggest that a turnout under 100,000 would be a wash-out for the cleric.
In Islamabad, organisers set up a makeshift stage on Jinnah Avenue, where around 5,000 people waited for Qadri to arrive a kilometre (less than a mile) from parliament, which is barricaded behind shipping containers.
His supporters say he gives 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," said Huma Nadeem, a 20-year-old college girl, in Islamabad.
The floodlit platform has been festooned with portraits of Qadri and bullet proof glass has been put in place to protect him.
Thousands of security personnel have deployed in Islamabad 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 militants 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.
Qadri wants a caretaker government to be set up in consultation with the military and judiciary when parliament disbands in mid-March, and is calling for reform so that "honest people" can be elected at polls due by mid-May.
"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 AFP on Monday.
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.
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.
Earlier in the day, an AFP reporter saw men, women and children piled onto the rooftops of buses, flashing victory signs in a five-kilometre convoy of tens of thousands of people growing in number after leaving Lahore on Sunday.
As they passed through towns and villages, they waved Pakistani flags, danced to drumbeats and were showered with rose petals by well-wishers.
"I'll stay there until real change comes or until Tahirul Qadri asks me to go back. If I have to stay 10 years, I'll stay there," said Hafez Aamir Chishti, a cleric from Lahore who joined the protest march on his motorbike.