The opposition parties in Pakistan on Thursday announced to field candidates for the office of the prime minister to challenge Imran Khan, whose Pakistan Tehrik-i-Insaf party emerged as the single largest party in the July 25 elections.

The National Assembly or the lower house of the parliament will elect the prime minister later this month.

In view of the majority seats in the National Assembly, Imran Khan will be elected as the new prime minister of Pakistan, PTI spokesman Fawad Chaudhry said.

But all opposition parties announced that they will field candidates for the office of the prime minister, speaker and deputy speaker in the National Assembly.

PML-N leader Ahsan Iqbal said that opposition parties will field Shehbaz Sharif, the PML-N president, as their candidate against Imran Khan.

Talking to reporters after the opposition meeting, Sherry Rehman, an opposition senator, said the opposition parties have formed a joint working committee to chalk out future strategy to give a tough time to the next government.

She said the opposition parties will also field candidates for the speaker and deputy speaker of the National Assembly.

The opposition leaders said they would continue protest against what they alleged "rigging" in the elections in and outside the parliament. However, they said they will remain part of the parliamentary system.