Hassan Rouhani openly questioned the actions of the Guardian Council, a conservative-dominated panel that vets all prospective lawmakers, after it said Monday that 60 percent of 12,000 election hopefuls had been excluded.
Reformists, largely sidelined from Iranian politics since the disputed 2009 re-election of hardline conservative president Mahmoud Ahmadinejad, bore the brunt of the vetting, with just one percent of its hopefuls winning approval.
Speaking in Tehran, Rouhani was warmly applauded when he suggested the public would see through steps that could amount to favouritism, saying it would dent the ballot's competitiveness.
"If there is one faction and the other is not there, they don't need the February 26 elections, they go to the parliament," Rouhani said, laughing but then scorning such a prospect.
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"No official without the vote of the people would be legitimate. Executors and observers should pay attention that the law is respected."
Rouhani's intervention came after the Guardian Council said only 40 percent of all candidates had been deemed eligible.
But the movement has regrouped since Rouhani, a moderate cleric, took office in 2013, raising hopes of a comeback.
The election vetting process has been contentious for months and on Monday the Guardian Council revealed its initial list.
Only 30 reformists from 3,000 applicants were approved, according to the movement's officials.
The poll, following Iran's nuclear deal with world powers, is widely seen an opportunity for reformists and moderates close to Rouhani to make gains against a currently dominant conservative camp in parliament.