Media playback is unsupported on your device Media caption Protest leader Dr Tahirul Qadri says the demonstration was "peaceful"

Pakistan police have fired tear gas at anti-government protesters marching on the official Islamabad residence of Prime Minister Nawaz Sharif.

The city's two main hospitals have told BBC 264 people have been injured, including at least 26 policemen.

Demonstrators loyal to opposition politician Imran Khan and cleric Tahirul Qadri have been taking part in a sit-in for two weeks.

They want Mr Sharif to resign, alleging corruption and electoral fraud.

Islamabad police chief Khalid Khattak told the BBC that close to 100 protesters had been arrested at the scene.

"Many of them were armed with axes, hammers and cutters, and I'm sure they also have firearms though we haven't seen one yet," he said.

On Friday Pakistan's powerful army chief, Gen Raheel Sharif, stepped in to mediate amid an ongoing deadlock.

Imran Khan ended talks with the government last week.

A government minister said that repeated attempts to resolve the two-week long standoff with protesters had failed.

Speaking to BBC World TV, cleric Qadri condemned the police's actions as an "unimaginable attack by the state upon the people", and denied that protesters were armed with weapons.

Image copyright EPA Image caption A number of protesters were seen carrying clubs as they marched on the residence of Pakistan's PM

Image copyright AFP Image caption Police fired tear gas to try to disperse the crowds

Image copyright AFP Image caption Nearly 100 protesters were arrested at the scene

Local TV pictures showed police throwing tear gas shells, and protesters throwing rocks back at them, some wielding sticks and slingshots.

An official at the Polyclinic hospital in Islamabad told reporters that the wounds of those injured were caused by teargas shells, stones and batons.

Two of the injured are reported to be in critical condition.

Analysis: M Ilyas Khan, BBC News, Islamabad

There is chaos on Islamabad's Constitution Avenue. There are clashes between the police and stone-pelting, baton-wielding protesters, some of whom have broken into part of the parliament and threatened other government buildings in Islamabad's red zone.

During the last couple of weeks, many in Pakistan and abroad have been fearing violence as a result of the protest campaign being led by former cricketer-turned-politician Imran Khan and Sufi cleric Tahrul Qadri to oust the 14-month-old government of Prime Minister Nawaz Sharif.

The violence not only jeopardises prospects of the country's fledgling democracy, which for the first time saw peaceful transition from one elected government to another, it also threatens to destabilise Pakistan months before Nato troops are scheduled to leave Afghanistan.

The Pakistani military has often intervened in similar situations in the past, but domestic compulsions such as the operation in North Waziristan and Western pressure on the army to refrain from becoming involved in political decision-making may keep its role limited.

Nevertheless, even if Mr Sharif's government survives this episode, it is going to come out rather bruised.

Military helicopters were despatched to survey the scene on the Constitution Avenue, one of the most secure zones in the city, housing a number of government buildings and residences.

The violence was sparked when the two opposition leaders ordered their supporters, many whom were wielding batons, to move closer to the PM's house.

The protesters began their sit-in after a huge march from Lahore to Islamabad two weeks ago, vowing to camp out in the capital until the government stood down.

Last year's elections marked Pakistan's first civilian transfer of power.