Pakistan's Supreme Court on Monday began hearing a raft of petitions against General Pervez Musharraf's plan to be re-elected as both president and chief of the powerful army.
The legal challenges come just days before the military ruler Musharraf is expected to file his nomination papers for a parliamentary ballot -- due before October 15 -- for another five-year term.
The petitions have been filed by the fundamentalist Jamaat-e-Islami party, the outspoken cricketer-turned-politician Imran Khan and an association of pro-democracy lawyers.
Musharraf has been beset by crises since his botched attempt in March to sack Supreme Court Chief Justice Iftikhar Muhammad Chaudhry, which sparked nationwide protests and a spate of judicial activism.
"The hearing of the petitions has begun," a Supreme Court official told AFP without elaborating.
The appeals all argue that Musharraf, who seized power in a bloodless coup in 1999, should not be allowed to hold his military and civilian offices at the same time.
They also oppose Musharraf's aim to be re-elected by the outgoing parliament and provincial assemblies, saying there should be a general election first to reflect the changing political landscape.
The former commando's plans for re-election have been the sticking point in negotiations with self-exiled former prime minister Benazir Bhutto on a power-sharing pact.
Bhutto, who has been living in Dubai and London to avoid corruption charges in her homeland, announced on Friday that she intends to return to Pakistan on October 18, with or without a deal.
Pakistani authorities a week ago expelled another ex-premier, Nawaz Sharif, when he tried to fly home to challenge Musharraf, the man who ousted him eight years ago.
As the political scene in the nuclear-armed Islamic republic of 160 million people hotted up, the opposition condemned "unconstitutional" changes in election rules that previously barred Musharraf from seeking a new term.
Pakistan's election commission said at the weekend that it had amended rules restricting public servants -- such as army chief Musharraf -- from standing in polls unless they have been retired from their jobs for two years.
"The election commission cannot amend or suspend any article of the constitution," said Siddique-ul Farooq, a spokesman for Sharif's faction of the Pakistan Muslim League party.
"They are trying to create confusion."
An alliance of key opposition parties also said they would quit parliament en masse if the commission accepted Musharraf's nomination. It includes Sharif's group but not Bhutto's Pakistan People's Party.
The commission is expected to announce the schedule for the polls this week.
MPs from the All Parties Democratic Movement decided at a meeting on Sunday to "resign the day the election commission accepts Musharraf's nomination papers," Farooq said.