Image copyright AFP Image caption Imran Khan was mobbed by supporters as he led a protest march against Nawaz Sharif at the weekend

Imran Khan's opposition party in Pakistan says it has decided to resign all its seats in the national assembly.

A senior member of party said they were trying to force Prime Minister Nawaz Sharif to step down.

The Pakistan Tehreek-i-Insaaf (PTI) party has 34 of the national assembly's 342 seats, making it the second-biggest opposition group.

Mr Khan accuses Mr Sharif's party of vote rigging in the 2013 election and has called on him to stand down.

Mr Sharif won that vote by a landslide.

Shah Mehmud Qureshi, a senior PTI leader, said the party would also withdraw from three out of four provincial assemblies in Pakistan.

The fourth province, Khyber Pakhtunkhwa, is under PTI control and officials there would not resign, he said.

The MPs' resignations must be submitted to the national assembly speaker but correspondents said this would not happen on Monday as parliament had adjourned for the day.

Mr Khan said he will join his supporters to march on Islamabad's "red zone" on Tuesday, an area of Pakistan's capital that houses key government buildings and foreign embassies.

The government has repeatedly said it will not allow protests in the heavily protected district and the BBC's Shahzeb Jillani says physical confrontation is likely if the march goes ahead.

"If the police use weapons to stop us, I will be the first to fall," Mr Khan told a crowd of supporters on Monday evening.

Image copyright AFP Image caption Mr Khan said he will march on Islamabad's "red zone", which is heavily protected by soldiers

Image copyright AFP Image caption Police estimated that 60,000 opposition supporters attended further protests in Islamabad on Monday

The cricketer-turned-politician has been demonstrating along with thousands of his supporters in the capital Islamabad since Friday to demand fresh elections.

But the BBC's Ilyas Khan says Mr Sharif's government is likely to ignore calls for a re-election.

Mr Khan's rally took place at the same time as a similar demonstration by anti-government cleric Tahirul Qadri, who told crowds that the protests must continue until they bring a "peaceful revolution".

Mr Sharif's victory was the first democratic transfer of power in Pakistan, which has a long history of coups.

Critics say Mr Qadri has close ties to Pakistan's armed forces, but he told the BBC on Monday that he has "never been in contact with the military establishment."

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.

Former president Asif Ali Zardari, co-chairman of the Pakistan People's Party, the largest opposition party, said such "unconstitutional means" would not help the cause.

"Democracy will not be served by calls for civil disobedience nor by a stubborn refusal by any side to engage in a meaningful dialogue on political issues," Mr Zardari said in a statement.