NAUDERO, Pakistan — Benazir Bhutto’s 19-year-old son was chosen today to succeed her as chairman of her opposition party, extending Pakistan’s most famous political dynasty but leaving the real power to her husband, who will serve as co-chairman.

Both major opposition parties also decided to run in parliamentary elections scheduled for January 8, apparently ending the threat of a wholesale boycott as Pakistan struggles to move to full democracy after years of military rule.

But earlier, a spokesman for the country’s ruling party said the vote may be delayed up to four months, claiming the parliamentary elections would lose credibility if held as scheduled. He expected a formal announcement within 24 hours.

“How long the postponement will be for will up to the Election Commission,” an information secretary of the ruling Pakistan Muslim League-Q, Tariq Azim, said. “I think we are looking at a delay of a few weeks … of up to three or four months.”

Bhutto’s Pakistan Peoples Party central executive committee met privately to choose her successor three days after the two-time prime minister was assassinated in a suicide attack that thrust the volatile Islamic nation deeper into crisis.

Her son, Bilawal Zardari, a student with no experience in politics, said he would remain at Oxford University, leaving his father, Asif Ali Zardari, who was officially designated co-chairman, as the effective leader of the country’s largest political party.

“The party’s long struggle for democracy will continue with renewed vigor,” Mr. Bilawal told a news conference. “My mother always said democracy is the best revenge.”

Supporters chanted “Benazir, princess of heaven” and “Bilawal, move ahead. We are with you.”

Bhutto’s grandfather was a senior figure in the Pakistan Muslim League, the party that helped Pakistan split from India and lead it to independence in 1947. Her father — Pakistan’s first elected prime minister — founded the party in 1967 and its electoral success since then has largely depended on the Bhutto name.

Mr. Bilawal said that Mr. Zardari would “take care” of the party while he continued his studies. Mr. Zardari then told reporters to direct questions to him, saying his son was at a “tender age.”

Mr. Zardari, who spent eight years under detention on corruption charges in Pakistan before his release in late 2004, is a party powerbroker who served as environment minister in Bhutto’s second government. He has denied the charges of large-scale graft during his wife’s rule.

He immediately announced the group’s participation in the elections but said another party leader, Makhdoom Amin Fahim, would likely be their candidate for prime minister if they won.