ISLAMABAD, Aug. 27 (Xinhua) -- Process for an upcoming presidential election in Pakistan started on Monday as three candidates filed their nomination papers.

The election will be held on Sept. 4 to replace incumbent President Mamnoon Hussain, who will complete his five-year tenure.

Those who submitted nomination papers included Arif Alvi of the ruling Pakistan Tehreek-e-Insaf (PTI), Fazal-ur-Rehman of the religious parties' alliance Muttahida Majlis-e-Amal (MMA) and Aitzaz Ahsan of Pakistan Peoples Party (PPP).

The major opposition Pakistan Muslim League-Nawaz (PML-N) and some other small opposition parties announced support to Fazal-ur-Rehman.

The opposition parties had earlier agreed to field a joint candidate but differences emerged over the nomination of the candidate.

The PML-N wanted the PPP to change Aitzaz Ahsan with another candidate as the party was unhappy at some of his statements against the jailed PML-N former leader Nawaz Sharif.

Opposition parties continued meetings on Monday to remove internal differences and support Fazal-ur-Rehman, who heads the Jamiat Ulema-e-Islam party.

The opposition parties could give a tough time to PTI if they agree on a consensus candidate, according to political watchers.

On their part, the ruling PTI and its coalition partners are confident of winning the election as they have the required number of the national and provincial lawmakers.

The National Assembly, Senate and four provincial assemblies are part of the electoral college for the presidential elections.

The session of both houses of the parliament has been summoned to meet at 10:00 a.m. local time on Sept. 4 for the election.