Pakistan's army said it deployed extra troops as requested by the government. Two Pakistani security officials said a total of 700 troops had been deployed to guard the "Red Zone." They spoke on condition of anonymity because they were not allowed to speak publicly about troop movements.
Pakistan's Interior Minister Nisar Ali Khan pleaded for calm ahead of the march.
"Violence can't be allowed to happen," he said. "What is this all we are showing to the world?"
The demonstrators have camped out in Islamabad in two rallies since last week. Khan and Qadri have vowed to keep up the sit-ins until Sharif resigns.
Khan, who heads parliament's third-largest political bloc, announced Monday that he and his supporters would march into Islamabad's "Red Zone." On Tuesday, he said on Twitter that he himself would lead the march and urged Sharif to resign.
Khan and Qadri both called on their protesters to be ready to march toward the "Red Zone," where Khan said they would make a "Tahrir Square" outside the parliament, referring to the famed square in Egypt that saw mass protests in 2011.
"Let us promise that we will remain peaceful," Qadri asked his followers. "No one will trespass into any building," Khan told his supporters.
Authorities have said they would not allow protesters to enter the "Red Zone" and asked the two opposition leaders to reconsider.
About 30,000 security troops have fanned out across the "Red Zone" and Islamabad ahead of the announced march, police said.
Amid the growing tension Tuesday, Pakistan's army chief Gen. Raheel Sharif also met with the prime minister in Islamabad, said Khan, the interior minister.
Khan's Tehrik-e-Insaf party has been complaining that Sharif's Pakistan Muslim League-N rigged last year's elections. Sharif has agreed to set up a judicial commission to probe the allegation but refused to step down.
Khan has said he won't go home without Sharif's resignation, setting up a possible violent confrontation.
The standoff has raised fears of political instability in this nuclear-armed country of 180 million, which has largely been ruled by dictators since 1947.
Sharif, himself overthrown in the 1999 coup that brought former army chief Pervez Musharraf to power, has been meeting with top advisers ahead of the rally. The government also has invoked a rarely used article in the constitution allowing the military to introduce martial law if needed.
The Islamabad rallies come as Pakistan's military is waging a major operation against local and foreign militants in the North Waziristan tribal region bordering Afghanistan. The military said it carried our multiple airstrikes Tuesday in the Khyber and North Wazristan tribal region that killed 48 militants.
Pakistan's tribal regions are off-limits to journalists, making verifying claims over airstrikes difficult.
Also Tuesday, a roadside bomb struck a school van with children and teachers in Khar, the main town in the northwestern tribal region of Bajur, killing five people, a government administrator said.
No group immediately claimed responsibility for the attack. The Bajur tribal region has seen several military offensives against the Pakistani Taliban and foreign militants in recent years.
Associated Press writers Anwarullah Khan in Bajur, Pakistan, and Munir Ahmed in Islamabad contributed to this report.