Supporters of Muhammad Tahirul Qadri, Sufi cleric and leader of political party Pakistan Awami Tehreek (PAT) gesture as they begin their march toward the capital from Lahore August 14, 2014.
Tensions rose in the Pakistani capital on Tuesday as an opposition rally prepared to march toward the Parliament and government headquarters in Islamabad later in the day and authorities beefed up security measures to prevent violence.
The demonstrators, who have camped out in Islamabad in two rallies since last week, are demanding that Prime Minister Nawaz Sharif step down over alleged vote rigging in the 2013 parliamentary elections.
The rallies with tens of thousands taking part are led by cricket-star-turned-politician Imran Khan and fiery cleric Tahir-ul-Qadri who have vowed to keep up the sit-ins until Sharif resigns.
Mr Khan, who heads Parliament's third-largest political bloc, announced on Monday that he and his supporters would march into Islamabad's so-called Red Zone, which houses embassies, the Parliament, government offices and the residence of the prime minister and the president.
On Tuesday, he said on Twitter that he would himself lead the march and urged Sharif to resign. Authorities have said they would not allow protesters to enter the Red Zone and asked the two opposition leaders to reconsider.
Instead of the march, "we request that Imran Khan and Tahi-ul-Qadri agree to talks with the government," said Ahsan Iqbal, a senior Cabinet minister.
"Let us sit and find a political solution," said Iqbal, who was appointed by Sharif to lead the talks with the opposition.
About 30,000 security troops fanned out across the Red Zone and Islamabad ahead of the announced march, the police said.
Mr 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.
Mr Khan, who is revered in Pakistan for leading the national team that won the 1992 cricket world cup, has said he won't go home without Sharif's resignation.
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.
Mr 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 has also 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 on Tuesday said it carried our more air strikes in the area, killing 18 militants.
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.
Two children, two female teachers and the driver were killed, said Asmatullah Wazir. It wasn't immediately clear if anyone else was in the van or if there were any survivors.
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.