Pakistan protesters march on parliament in red zone
Anti-government demonstrators in Pakistan are advancing towards parliament, breaching a designated secure zone in the capital Islamabad.
Protesters used wire cutters and cranes to move shipping containers barricading the so-called red zone, which houses state buildings and foreign embassies.
They have encountered no resistance so far from security forces, who have been instructed to avoid violence.
The demonstrators want Prime Minister Nawaz Sharif to resign.
Thousands of anti-government protesters have been occupying two Islamabad highways.
Imran Khan, the cricketer-turned-politician, has been leading one group of protesters since Friday.
He accuses Mr Sharif's PML-N party of vote-rigging in the 2013 election and has called on him to stand down.
Mr Sharif's party won that election by a landslide in what was Pakistan's first peaceful transfer of power between two civilian democratic governments.
Anti-government cleric Tahirul Qadri has also mobilised his supporters to march on Islamabad.
The government has accused the protesters of attempting to derail democracy. They have offered talks with the demonstrators, but these have been rejected.
The protesters are advancing at a snail's pace, and are not likely to reach the parliament until late in the night, the BBC's Ilyas Khan in Islamabad reports.
Thousands of policemen are deployed around the red zone, and appear to be equipped with riot gear and teargas shells, but not firearms, our correspondent adds.
At the scene: Ilyas Khan, BBC News, Islamabad
The protesters chanted slogans as they breached the barriers to enter Islamabad's high security zone. The police offered no resistance, though they did steer the crowd through a single route instead of allowing them to spread out.
So tens of thousands of protesters are now set to occupy Constitution Avenue, which many believe will paralyse key state institutions, such as parliament, the Supreme Court and the federal ministries.
The unrest has seen the government of Prime Minister Nawaz Sharif put on the back foot; first it decided not to resist the protesters' convergence on Islamabad, and later conceded their demands of setting up protest camps where they wanted.
Now that they are inside the capital's "red zone", many fear the government's options to defuse the crisis may be shrinking.
Earlier on Tuesday, Interior Minister Chaudhry Nisar said: "The government has decided to hand over the security of the red zone of Islamabad to the army."
Some observers said the move could indicate that the government had the support of the military.
Fear of violence
Supporters of Mr Khan and Mr Qadri are angry about Pakistan's poorly performing economy, growing militancy, and the government's failure to deliver services such as a steady electricity supply.
But other opposition figures have criticised the demonstrations and Mr Khan's call for people to stop paying tax bills in protest at the government.
In a country with a history of military coups, there is a fear of violence and of a possible army reaction, correspondents say.
On Monday MPs from Imran Khan's PTI party vowed to quit their national assembly seats.
The Pakistan Tehreek-i-Insaaf (PTI) party has 34 of the national assembly's 342 seats, making it the second-biggest opposition group. The lawmakers have yet to tender their formal resignations to the speaker of the national assembly.
The party said it would also resign from seats on provincial assemblies, except for Khyber Pakhtunkhwa province, which it governs.