Pakistan’s Opposition demanded on Friday that Prime Minister Imran Khan and his administration resign within two days, raising the stakes in a protest campaign that the government has denounced as a threat to democracy.

The campaign is the first concerted Opposition challenge that Khan has faced since he won a general election last year promising to end corruption and create jobs for the poor.

“I’m giving a two-day deadline for the resignation,” the leader of the campaign, religious party chief Fazl-ur-Rehman, told a rally of tens of thousands of supporters in Islamabad.

The Opposition says Khan’s government is illegitimate and is being propped up by the military, which has ruled Pakistan for about half of its history and sets security and foreign policy.

The military denies meddling in politics and Khan has dismissed the calls to step down. Rehman, leader of the conservative Jamiat Ulema-i-Islam-Fazl party, is a veteran politician who can mobilise significant support in religious schools across the country.

He was joined at the rally by leaders of the country’s two main Opposition parties.

He had earlier warned of chaos if the government does not step down, but on Friday told the crowd they would decide what action to take if their two-day sit-in at the rally site failed.

He said he did not want confrontation with the military. “Now this government has to go but we don’t want a collision with institutions,” he said referring to the military.

“We want to see the institutions being impartial.”

Police estimated 35,000 people attended the rally.

Security was tight in Islamabad with the government and diplomatic sector — just a few kilometres from the rally site — sealed off, roads blocked by barriers of shipping containers. Schools were closed, public transport suspended and Internet services interrupted in some areas.

The army chief, Gen. Qamar Javed Bajwa, earlier urged Khan to handle the protest peacefully and avoid violence by both sides, a military spokesman said.

The government has said the protesters will not be allowed to paralyse the capital. Violence could erupt if crowds try to force their way into the sealed-off government sector.

A government spokeswoman said the Opposition was talking of protecting democracy but was a threat to it.

“These steps ... are against the rules of Pakistan and tantamount to destroying democratic norms and the constitution,” the spokeswoman, Firdous Ashiq Awan, said on Twitter.