Pakistani police have fired tear gas at thousands of protesters as they tried to march towards the prime minister's home in the capital, Islamabad, blanketing the route with clouds of white smoke and scattering demonstrators.
Police also fired rubber bullets to disperse the crowd, injuring some protesters, and at least a dozen were taken to a nearby hospital, police said.
The march's leaders, cricketer-turned-politician Imran Khan and anti-government cleric Tahirul Qadri, called for the demonstration to move to the house from separate massive rallies in front of the parliament, where they have been staging a sit-in for days.
Some 20,000 police in riot gear were charged with blocking the procession.
In speeches, Mr Khan and Mr Qadri said they will remain peaceful and urged security forces to abstain from using force against the protesters.
The two allege that Prime Minister Nawaz Sharif won the 2013 election due to massive voter fraud. They demand he step down but Mr Sharif has refused.
Both Mr Khan and Mr Qadri, a dual Pakistani-Canadian citizen with a wide following, also demand reforms in Pakistan's electoral system to prevent future voter fraud.
Backed by parliament and many political parties, Mr Sharif has said he will not step down. Government negotiators are trying to convince Mr Qadri and Mr Khan to end their protest and abandon the demand for Mr Sharif's resignation.
The demonstration began with a march from the eastern city of Lahore on the country's Independence Day, August 14.
Mr Khan and Mr Qadri had called for millions of protesters to join but crowds have not been more than tens of thousands.
The protesters' presence and heightened security measures have affected life and badly harmed business in the capital. The rallies have remained festive, with families picnicking and men and women dancing to drums and national songs.
Riot police initially showed restraint to today's march but when the crowd started removing shipping containers used as barricades they fired tear gas that forced the crowds back.
TV footage showed protesters, including women and children, scattering in retreat.
Some fell to the ground and dozens were being treated in a hospital. Many, including two children, were shown being treated for the effects of tear gas.
Police official Farman Ali said the injured have been moved to a government hospital.
"They fired tear gas shells at us," said Ahsanullah Fakhri, 28, who was bleeding from his leg, as he exited an ambulance with some seven other protesters who had multiple minor wounds.
"I think they are also firing some bullets, I think rubber bullets," he added.
Mr Khan described the police action against the crowd as illegal.
"Now we will show this government, we will call for countrywide agitation and we will jam the whole of Pakistan."
Interior minister Nisar Ali Khan quickly visited the scene to boost police morale.
He said: "A group wanted to capture the prime minister's house and other buildings. We are under oath, and the police as well, to protect the state assets."