Pakistan's top military commanders convened a special meeting Sunday night after clashes between police and opposition protesters seeking to topple the government left three dead and hundreds injured.
The meeting, brought forward a day, came after violence broke out Saturday night when thousands of supporters of politician Imran Khan and firebrand cleric Tahir-ul-Qadri tried to storm Prime Minister Nawaz Sharif's house using cranes to remove barricades.
The fighting continued Sunday between police in riot gear and a few hundred protesters. Many protesters came armed with batons and slingshots loaded with marbles.
Shipping containers were set ablaze, several vehicles were torched, and hundreds of tear gas canisters lay strewn on the ground on Islamabad's normally pristine Constitution Avenue after almost 24 hours of battle.
By nightfall on Sunday protesters were preparing for fresh clashes, breaking up the road to use chunks as missiles and preparing crude gas masks from cloth and plastic bottles.
Opposition groups marched to the capital on August 15 demanding Sharif's resignation, triggering a crisis that has raised the spectre of military intervention.
The military said army chief Raheel Sharif had called a meeting Sunday of top commanders at the army's headquarters in the neighbouring garrison city of Rawalpindi.
The prime minister has also convened an extra-ordinary joint sitting of parliament on Tuesday after presiding over a high-level meeting, a cabinet minister told AFP.
"The meeting strongly condemned the desecration of state institutions and declared it undemocratic and unconstitutional," the minister said.
Opposition leaders claim the 2013 election which swept Nawaz Sharif to power was rigged, though local and foreign observers rated the polls as relatively fair credible.
Speaking from the roof of a shipping container Sunday, Khan said: "Now I ask all Pakistanis: rise up against this government. This is not a constitutional government -- they are killers.
"We will continue until our last breath. I urge all Pakistanis to come out," the cricketer-turned-politician said, adding he would file murder charges against the prime minister over the violence.
The security situation has deteriorated in the capital.
Small groups of protesters blocked the main expressway linking the city to the airport and forced traffic to a halt by putting tree branches and rocks on the road and pelting vehicles with stones.
A group of up to 15 teenage protesters armed with bamboo sticks and rocks attacked a vehicle carrying three United Nations staff members near the protest zone.
"Their vehicle was damaged but they escaped unhurt," a UN source told AFP.
In Lahore around 300 Khan supporters burnt tyres and attacked the Metro Bus service in the Model Town suburb where the prime minister has a private residence.
- Military intervention? -
The Pakistan Institute of Medical Sciences reported three deaths -- two men from injuries sustained in the fighting and another who died of a heart attack.
At least 481 injured people, including at least 118 women and 10 children, were rushed to the city's two major hospitals, according to officials. At least 92 police were among the wounded.
The crisis took on a new dimension on Thursday after it was announced General Sharif would mediate.
Observers believe that if the prime minister survives the crisis it will be at the cost of significant concessions to the army -- including allowing ex-army chief Pervez Musharraf, currently on trial for treason, to leave the country.
But political analyst Mosharraf Zaidi said the protests had become difficult to contain and the opposition could be trying to get the military more directly involved.
"The single objective is to force Sharif to resign and possibly force the military to intervene," he said.
Zaidi also dismissed claims by the protest groups that they had not initiated the violence, saying: "You cannot be in attack formation and then claim you had no role in fomenting this violence."
Javed Hashmi, a senior member of Khan's Pakistan Tehreek Insaaf (Movement for Justice) party, broke ranks on Sunday, saying the protest had gone too far.
"How can I side with you when you march on the parliament? There is hardly any distance left between us and martial law," he said as he addressed a press conference.
The protest leaders have drawn thousands to the streets of Islamabad, but their call has not mobilised mass support in a country of 180 million people.
But many of the protesters are now openly calling for the military -- which has ruled Pakistan for half its existence -- to return to power.
"Last night, the police used tear gas and rubber bullet against us. Now we are waiting for the help of the army. We are hopeful. Insh'allah (God willing) the army will save us, they will do something for us," said a female protester in her twenties called Ammara.
Sharif, whose previous term as prime minister ended in a bloodless coup in 1999 by ex-ruler Musharraf, has vowed to stay on.