Pakistan Presidential Election Oct. 6

In this picture released by Pakistan Press Information Department, President Gen. Pervez Musharraf, right, listens to Prime Minister Shaukat Aziz, left, during a meeting in Rawalpindi, Pakistan on Tuesday, Sept. 18, 2007. Musharraf will step down as army chief and restore civilian rule to Pakistan, but only after he secures re-election as president, a government lawyer said. (AP Photo/Pakistan Press Information Department/HO) (AP)

"The letter and the spirit of the constitution do not permit President Musharraf to be a candidate," said Sadiq ul-Farooq, a leader of Sharif's party, the Pakistan Muslim League-N.

The resignation threat, echoed by the religious parties, could deny the presidential vote legitimacy and prolong Pakistan's political uncertainty.

Opposition parties called for nationwide street protests on Friday, and Ali Ahmed Kurd, a senior lawyer, said protesters would besiege the Election Commission office and "tear up" Musharraf's nomination papers, which must be filed Sept. 27.

However, the biggest threat to Musharraf's plan appears to come from the Supreme Court, whose independent-minded chief justice survived a botched attempt by the government to fire him in March.

On Thursday, the court rejected one opposition complaint against changes to the election rules that favor Musharraf. It is still considering a raft of broader petitions contesting Musharraf's eligibility for another presidential term and a verdict is expected within days.

Several of the judges complained Wednesday that lawmakers were trying to place too much responsibility on the court.

"If (Musharraf) has breached the constitution, then parliament should impeach him," Justice Mohammed Nawaz Abbasi said.

Najam Sethi, editor of the Daily Times newspaper, said most observers think the court will not disqualify Musharraf completely, although it could force him to step down as army chief before he stands for re-election.

Bhutto, who plans to return to Pakistan from eight years of self-exile Oct. 18, has threatened to join the opposition boycott unless the government drops corruption cases against her and other politicians and lifts a prohibition on her serving a third term as prime minister.

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Associated Press writers Sadaqat Jan and Matthew Pennington contributed to this report.